... but rewarding comments is a relatively rare action...
Some users buy votes for good comments from bots because their own vote is too low. With a donation button, they can easily reward these comments with small amounts.
An easy to use donation button, next to the other buttons, is missing in all frontends.
Out of all the organic votes, they are relatively rare.
I'm all for a tipping button, this has been discussed multiple times and likely will eventually be implemented.