That might be because most modern games are much more complicated than Doom. It’s also possible that modern game designers don’t want people to abandon the game before having a chance to really get what the core of it is about.

Don’t get me wrong, there is still a place for games with mystery and obscure mechanics etc, but it doesn’t make sense for most games to be like that. It’s just an unnecessary way to keep away players that would otherwise love the game.