There should be a reason skill is almost never involved in success.

In my understanding, this reason is network latency. I think you need low latency to make an action game where achievement is dependent on skill.

Skill isn't particularly related to success in most single-player games that allow leveling/improving equipment. The developers want to please their paying customers, so they will do their best to prevent a situation where someone isn't skilled enough to complete the game. Since there are usually only a few game endings, everyone gets to see the same result, and so their playing skills don't ultimately matter. Adjustable game difficulty serves the same end.

Sure, some games are hard enough that not everyone can beat them, but these are the exceptions and they can even become famous for that quality. (Anecdotally, I remember reading claims that Japanese games are much more likely to be unbeatably difficult than are Western ones.)