It's just a general thing that 90's music was just bad, especially for music. Bands that were good put out music that wasn't very good, and new artists only contributed to how awful the 90's already were. Even 90's-era sports cars looked like stuff. There are few good things out of the 90's, mainly the Power Rangers, DOOM, DOOM II, Duke3D, Shadow Warrior, Wolfenstein, and Nintendo.
Nirvana wasn't an amazing band, but you seem to only be focusing on them. The 90s were great. TV shows were great, cartoons were great, games were great, and even the music was great. There was Full House, Hey Arnold!, the N64, and bands like Korn and SOAD were still playing clubs. If GTA:SA is accurate about the 90s, then I sure did like the cars from then.
You can't compare every metal band from '90-now with Metallica just because they're what you prefer. A lot of music nowadays doesn't have that raw sound you'd get from a pre-y2k song, but they still sound great. Even in the 90s, a lot of bands had a sound that seemed different than what came before it. Pop music started to sound more 'clean' (if you get what I'm saying) while most rock/metal got heavier.