Aside from the fact that he's got almost as much money as God, Jerry gets paid VERY handsomely for one-night appearances for both the occasional public event and for corporate (private) events... one show nets him more than most people make in a year. It goes without saying that if he wanted to he could get another series or a movie deal or anything... I seriously doubt he WANTs to do another long-running series anytime soon. And there are also his book(s) which I'm sure get him an advance in the millions of dollars too.
When you have that much money, you work when you WANT to, not because you have to.
As for the other Seinfeld folks, though they're surely paupers by Jerry's standards, no doubt they each have more liquid assets than most people accrue in a lifetime. No doubt you'll see some of them try other TV pilots, but really, work is a choice not a necessity for them too I imagine.
Besides, all four of them are probably going to be enjoying very healthy residual checks for the rest of their lives.
Palomine practices what he preaches. Beloved reason, indeed.
I know for a fact that Jerry has been courted and lured by every network in town, but has yet to find something in which he is sufficiently interested. He is also involved in a number of successful ventures with American Express and other advertisers, and as Palomine suggested, probably made more money today than most of us will make in a lifetime.
Seinfeld was a unique enterprise. Jerry is being wisely selective, a luxury he has handsomely earned.