Because on Netflix what you're skipping is something you expect and maybe even want to see. On YouTube it's ads, which you don't necessarily expect nor want to see. And the skip button is just associated with that.

Also, on YouTube you sometimes have to skip multiple consecutive ads, sometimes you have to wait for a few seconds, and sometimes you can't skip them at all. It's not made to be practical, it's made to serve you as much ad-time as possible, as that is the bread and butter for YouTube (contrary to popular belief they don't make a lot of money, so it makes sense. Although I'm not sure I like they way they try to make money to stay in business).