I like the show and can't speak for the American demographic, but I can tell you that while Friends has universal appeal across many countries, regions, languages and culture, Seinfeld is extremely American, extremely New York, and extremely Jewish. I like it, but it's a lot less universal than Friends is. While Friends has broad appeal across Europe, Asia and Africa, Seinfeld makes fun of cultural norms specifically tied to the US and to a lesser extent Western culture in general. So not every joke will land.

Then you add the fact that every show since then has copied it in some way or form the past 3 decades, and it has an entire TV tropes page dedicated to that phenomenon of "seinfeld is not funny"