I suppose they thought they didn't need him anymore...they knew it all. Which is why the White Album is mostly crap with a few good songs here and there.
Early in their recording careers they listened to Martin when he told them a song was either bad or needed redoing. When they ditched him they allowed their rather large egos to make decisions for them.
Now, it is possible they simply ran out of good ideas by the time they split up. But I would bet getting rid of Martin didn't help. Certainly, McCartney's mostly crappy albums post-Beatles could have used Martin's musical expertise.
It's ironic that the only Beatle who I thought had consistently good music post-breakup was Harrison. He was the Beatle whose music Martin relegated to third place during the Beatles reign at the top.
Especially All Things Must Pass, his triple album release of 1970, produced by Phil Spector. By that time Harrison had compiled quite a backlog of great unrecorded songs due to being relegated to (pretty much) only one song per album when he was with the Beatles. He must've felt like he was just let out of jail.
Martin did produce a few of Paul's solo albums, and he did produce Paul's theme to "Live and Let Die."