Good ol' Sturgeon's law and the survivorship bias, right? A part of it probably also has to do with people generally only watching shows they enjoy in the first place — they probably don't have as strong an impression of shows that they only gave three episodes to if any shot at all, and that's what makes it easier to forget just how much crap there was in the old days. And this is not to mention that people might just not have had as "refined" or "picky" tastes when they were (presumably) first getting into anime in the 2000s versus today, when choices abound in what to watch — or how the people who say that anime was better in the 2000s are disproportionately likely to just be misogynistic otaku weirdos to begin with.

Incidentally, the average year of release of all the anime I've watched, by hours watched, is 2007~2008: 1 hour of anime from the 1960s — 18 from the '70s — 35 from the '80s — 112 from the '90s — 225 from the '00s — 506 from the '10s — 120 from the '20s.