Any fan of Seinfeld will tell you Seinfeld is one of the least funny characters on the show. His stand up bits are more like monologues for the episode than gut busting comedy routines.
A huge part of Jerry's humor and appeal is his timing and delivery. A lot of what he says isn't that funny, but timing is everything.
George as a character and how Jason Alexander delivered that character is a once in a decade level of incredible. Coincidentally, I have just started re watching Seinfeld again, starting at season 2 and I have a new found appreciation of the show. I used to think Kramer and Jerry were the funniest when I was younger and I now realize George, Elaine and the rest of the auxiliary cast are the highlights. The first time Jerry and Elaine go to visit Jerry's parents in Florida...fucking brilliant.
Lastly, having lived in NYC for seven years now there are a lot of jokes that only registered after have had living here. That isn't to say Seinfeld isn't funny unless you have lived in NYC, but the are jokes that are elevated when you have a personal experience to reference, much like all great comedy.
To say "Seinfeld isn't funny" is nothing profound. It is a show with legendary hype, a perfect time capsule of its era and as with any comedy, isn't for everyone. There is always a fine line of distinction when it comes to judging predominantly subjective mediums, but I can say that if an objective measure of comedy was applied to Seinfeld, I would confidently declare it is a funny show. I am sorry you don't enjoy it.