Because they're not that good of a band. They're fine. They don't deserve anywhere even remotely near the praise they get. Sure they're the premier grunge band but that aint saying much. Talk about an inconsequential flash in the pan genre.
Grunge was a genre for its time and its place, but that's where it should stay. I mean what other truly meaningful impact on music did grunge have other than being the spiritual predeccesor to 2000's butt rock? The hollow shell of rock music that is post-grunge that we all had to suffer through and really haven't recovered from since. Yea ya know all those bands that sound like something Trey Parker created as a joke? That's the legacy grunge left. Bleck.
I think it's fair to assume you're Gen X? I totally understand why you feel this way about them but no one outside of that specific period of time gives a sheet about grunge. Nirvana's ok, I don't hate Nirvana, but we need to stop pretending like they're some transcendent legendary band.
As for the "lowest point in human history" that really just sounds like an "Old Man Yells At Cloud" situation. Big "back in MY day!" energy. Huge "get off my lawn" vibes. There's tons of good music, TV, movies, art out there. Like every other period in time, the popular stuff is rarely the good stuff, but it's always out there.