Notice that if Seinfeld (the tv show, not the person) has been created and aired in the last 8-9 years, the show would have been not so funny? And it would possibly be a gigantic twilight flop.
For example, in season 3 episode 40, Jerry gave his spare keys to Elaine, then when he needs them, he goes with George to Elaine’s (who has her keys) to search for his spare set. All because he failed to reach her apartment/landline from a public phone.
So with mobile phones, this episode will pretty much end with Jerry contacting Elaine on her mobile phone. That’s one boring plot.