Consistency for the sake of consistency is a dangerous notion.
Illustrator, as any other app not only within Adobe suite, has many entry points, and not all of new users come in it from Photoshop. Some come from Corel Draw, Xara, Canva and Figma (I mean it). Their expectation will always differ, and there is no true way to ease the learning curves for all of them.
In fact, making it more friendly for newbies often hurts seasoned veterans, and vice versa.

Funding probably influence this too, but in a veeery distant manner. Some great product and plugins are made with a very limited funding. Besides, discussing that would always be a speculation without the actual data — and I doubt we ever get it.

Speaking of Standart — well, I agree.
Photoshop was released in 1987. InDesign — in 1999. Illustrator started in 1985, got shipped in 1987.
Peers at least, right?

And I don’t agree with the notion it’s lacking 20 years, hardly. I can name you many things Ps was lacking for many years, while Ai had been having them for years. But it won’t get us anywhere. The opposite is also true, and these are just two very different apps, they work with different media (although the borders blur more and more), and they have their own legacies, traditions, habits, and angry users.

Take modifiers. In Ps, to duplicate a thing, you have to hold Opt/Alt before you click — but then you can release the key while still dragging.
In Ai, you have to hold the modifier until you release it, but you don’t have to hold it before the click.
BOTH methods are valid and have benefits. But only one can be a default one. Making it an option takes time and demand. And UV is a way to measure it (there is actually a request for that here, you can find it if this is something you need).

Comparing user count is also hardly fair approach :) It fluctuates and it’s a two-way factor.

Anyway, Ai is a complex thing. But it can allows many things. And balancing it all is not a simple feat.
It does not mean it’s PERFECT, hell no! :D

That why UV is that important. So if you have some other things you spot and wish to be changed — don’t hesitate to upvote and request, report and argue. This helps a lot.

Egor;
Thanks for the update and great information. Of course if you ask 3 different users how they like the UI you will get 3 different answers. However as large as Adobe is and the fact they are the "Industry Standard" they should, have their apps share commonality. Heuristics should almost be seamless between PS, AI, ID . . . Having been a long time user of PS I am only now grasping navigation in AI.Albeit I have only been using AI for a couple months now. I understand it is a resources issue on which program gets the funding. AI is really where PS was almost 20 years ago. Yes, there are far more users in PS than AI. I believe a lot of this can be attributed to the very steep learning curve with AI.

Doble-click on a tool icon is a common way to open a tool’s option or dialog in Illustrator. Once you learn it, you just use it.
But yes, despite this approach is several decades old, it's rarely used today and new modern users won’t know about it.

Illustrator recently added Dimension tool that has a ton of options. To inform new users about them, they added a 'gear' button into the dedicated floating panel with sub-tools. This can hardly be done for other tools without sub-modes (most of them), and even sub-tools are considered to be a questionable approach.

There is a dedicated request to list all the options for the active tool in a separate panel: http://illustrator.uservoice.com/forums/333657/suggestions/44146278
and another more arrow but more popular request to add all Blend options into similar panel: http://illustrator.uservoice.com/forums/333657/suggestions/31988758 — or a dialog, which means different people prefer different ways.

Dialogs and panels are two main ways Ai uses. Both have their pros and cons.
Some dialogs in Ai should be panels, some other better stay as dialogs...
A general request to eliminate modal dialogs can be found here: http://illustrator.uservoice.com/forums/333657/suggestions/44131527

Speaking of symbol sets.
This Symbolism toolset (Symbol Sprayer tool group) is rather old, it was introduced in version 10 in 2001, and was never really improved since then.
It never had its own submenu in Object top menu, has zero dedicated commands it the contextual right-click menu.

But some things you ask for are doable.
You can convert a Symbol Set into a group of symbols with Object > Expand command (but can’t covert them back into a set!)

As for the task of scattering some objects along a path — the team is aware of the problem and is reviewing options to solve it.
There is a dedicated request to Paste / repeat symbols / objects along a path: http://illustrator.uservoice.com/forums/333657/suggestions/41707711

For now I can offer workarounds the community has developed over the years to do that:

1. A script by @peprintenpa: an article (https://sppy.stars.ne.jp/alongpath) and a shop page (https://tawami.booth.pm/items/4029590) — supports rotation, a paid one

2. Distribute On The Path (https://shspage.blogspot.com/2014/02/distributeonthepathjsx.html), a free script by Hiroyuki Sato (originally developed and discussed here — https://community.adobe.com/t5/illustrator-discussions/need-a-developer-for-script-distribute-symbols-on-a-path-and-rotate/td-p/5879720) — does not support rotation

3. Rotate Toward Point (https://shspage.blogspot.com/2014/02/rotatetowardpointjsx.html), a second part of the solution above

4. Dup at Selected anchors (http://shspage.com/aijs/en/#dup_at), another free script by Hiroyuki Sato, does not distribute evenly, but uses points of the path as positions

5. FindReplace Art plugin by Astute Graphics (https://astutegraphics.com/plugins/findreplace)

6. Replace With Top Object command by Graffix (https://rj-graffix.com/toolshed-adds-bracket-tool-plus-replace-art-and-bust-up-paragraphs/)

Then, there is a request to have a scatter brush with more than one custom elements: http://illustrator.uservoice.com/forums/333657/suggestions/37324495

And these solutions currently exist:

1. Live Symbol Path effect from CValley FILTERiT suite: https://www.cvalley.com/products/filterit5-2/ (sadly, the site is offline since recently)

2. Symbol Stipple effect for Astute Graphics (https://astutegraphics.com/plugins/stipplism)

3. Make symbols work in brushes (https://illustrator.uservoice.com/forums/333657-illustrator-feature-requests/suggestions/36364858-make-symbols-work-in-brushes)

Thanks for the request anyway! We need more questions like this to get this moving.