Why hasn't there been a three-peat in the NFL (specifically the Super Bowl era)?

I mean technically there was the Jim Kelly with 4 straight SB appearances but unfortunately, he lost all of them. But 4 straight is still insane.

The closest in terms of actually winning was Mahomes but he got embarrassed (lol). Montana and Aikman had a chance but their teams lost in the playoffs.

But my question is as a new fan of the NFL, how come there hasn't been a team that has won 3 straight? Like why didn't the Patriots with Tom Brady three-peat?