Probably because the algorithm steam uses to promote games doesn't throw the crap stuff on the front page most of the time. So no one feels like they're paging through tons of spam to find the game they're looking for.

The flip side is that means if you're new, and write a good game, you better advertise it yourself, or it will die in obscurity.

These things regularly come up, and people complain about the obvious garbage, that constantly fill their store pages (which doesn't really happen to me).

They dont get a pass and they should absolutely be moderating it better.