Why even use any of the dozens (if not hundreds) of options available to you, which you painstakingly earned and chose over the course of the game, when you can just use one of the base actions available to you from the start of the game to kill the "boss" with one hit?
I could think of few reasons ...
1) Its aproximately million times more fun.
(Wich alone should be reason enough in my honest opinion)
2) If you shove your enemy into the lava sometimes you loose their loot this way.
(Should be every time in my honest opinion)
3) In SOME cases it can be more effective ... sure, you can "insta kill" with shove, presuming your roll will end well and your enemy will stand close enough to the edge ... but with some of those "other options" you talk about can sometimes kill more than one enemy ... that is something Shove can never do.
4) They dont exclude each other.
(While they probably should ...)
Since Shove is Bonus Action, and all (or at least most) other attacks are Actions, you can easily do both ...
Sure i know this is just another argument for Shove being unballanced mechanic ... but the question was "why not use base actions" ... so, this answer is legit. :P
I really thought i came up with more reasons ...
But it seems like this is it for now.
