Third-party developers can add custom items in menus.
Some create their own sub-menus, some add them in-between native commands... there are several practices they use and when they all get used together it feels somewhat chaotic.

But it feels like this is because plugin devs can’t fully control the commands in menus. It turns out, they can’t control the order and sorting, add dividers in Effects (but seemingly can in every other menu)...

Effect menu is the most weird menu in Ai, it looks different from others. Maybe that’s why?

There is an existing UV post about it:
https://illustrator.uservoice.com/forums/601447-illustrator-desktop-bugs/suggestions/36951433-effects-menu-is-smaller-larger-than-other-menus-on