Why is there no UNDO button?

[mawzzz]

All (or almost all) games have it. Misclick - or - realizing you use a card for placement or bonus - or - simply changing your mind when you see the outcome. Not if you get new information of cause.

Its very frustating.

[Meeplelowda]

All (or almost all) games have it. Misclick - or - realizing you use a card for placement or bonus - or - simply changing your mind when you see the outcome. Not if you get new information of cause.

Its very frustating.

This is a great summary that I can't improve upon written by a BGA developer about adding an undo function on BGA.

Why are the inclusion of undo buttons so random and sporadic over the games on here?

Undo needs to be implemented individually for every game. It's not a box you check to just make it happen. Furthermore, undo is tricky to implement correctly and very bug prone. As such, it's often avoided by game developers.

Either commit to allowing rectons to be made, or ban them over the entire site unless a game specifically allows for it in the rules.

Wish granted?
Official BGA policy states that undo should be avoided in games.
"As a rule of thumb, on BGA we advise you to not undo moves. Undo are painful for opponents, in real world and online. Some players may also use Undo to 'test' situations while they should just think instead. In many cases, proposing an 'Undo' means also adding an extra step, which is not good for the game interface."
See https://boardgamearena.com/doc/BGA_Undo_policy

Since BGA seems to be going to allowing them, why doesn't this game have one?

In summary:
1) Undo isn't encouraged
2) And it's not trivial to implement

I developed Gaia Project and got a similar request. I don't want to volunteer a dozen hours to implement a feature that I don't feel is needed and may introduce tons of bugs.