Because it's really not necessary and your suggestion is more complicated that it sounds.
Even some of the tags that have light content is just because it hasn't been fully implimented and is planned later on. Most of what you suggested can be solved simply by using the search function. The real issue is that people seem to be incapable of actually using it. And no it's nothing like politics because this is entertainment and not something that effects your personal lives the majority of the time.
It's like you saw a movie you disliked. Yeah it was bad and you disliked it so you choose to never watch it again and go do something else.
If you want to find out whether content is heavy or light use the search function ask people in the threads, look at reviews. This stuff takes at best a couple of mins, but you people just choose to instantly download instead of doing the bare minimum of research before getting into something. The site could use more tags I agree but having some sort of tag for light or heavy content is pretty useless when that information is usually on the front page.
You people need to do more research before you commit to something. And stop getting mad when people point it out. This is not like who you vote for this is like choosing what movie you want to watch on movie night. If you don't know what that movie on Netflix is about you can use google or watch the trailer.
Interesting point about planned content. I'm genuinely curious how many long-time players of Power Vacuum have been turned off with the gradual addition of more niche content.
Pr0GamerJohnny's point about politics may not have come across, but your all-or-nothing stance sounds
exactly like politics (in the US).
Movies are a good example of a tiered approach though. Compare two hypothetical movies rated "PG-13" & "R" (again, in the US). Both films may include violence and profanity, but you can expect these elements will differ in frequency/intensity based on the ratings.
Reviews are also inherently
subjective, so the idea of a content rating system for porn games has
some merit. Perhaps qualifiers like "brief/occasional" vs "extreme/frequent" could work?