On stackoverflow it's always the least upvoted answer that solves the problem most of the time. Why

Probably because even if it apparently solves the problem, it is poorly optimized, is bad practice, doesn’t consider edge cases or something similar.

Or you have an asshole of Questioner who asks gets three answers (one was mine) posts his own answer (there was different ways of solving the problem) and then proceeds to downvote every answer except his own

@Lensflare not necessarily. Most recent answer can be more up-to-date than the accepted answer from 10 years ago.

@WildOrangutan That’s why I didn’t include "is outdated" in my list.

This definitely doesn't match my experience.

But for some topics, answers can become obsolete over time. New relevant answers naturally start out with no upvotes. So in that case there has to be a timeframe where the best answer has less upvotes than the outdated one. That is why StackOverflow allows to show recently upvoted answers first.

For a while, I was sorting by most recent because of exactly this. The trending sort helps with this a bit, but not perfect.