Not all basses have neck dive, but the ones that doOk, I did not bother testing while seated because I’m pretty sure any object will tip one way or the other while perched atop my leg. I also never play without a strap and rarely while seated.
I just tried this with three basses and two straps. Two jazzes and one stingray. They all behaved the exact same way, when positioned anywhere between roughly 50 degrees upwards (totally scientific estimate) and 0 degrees parallel with the floor, they all stayed exactly at the orientation I moved them to. Didn’t have to hold up anything with my left hand. In fact, I kept my hands off the basses except for initially positioning them.
So either my collection defies physics or y’all are doing something different.

My last and most egregious offender was my Dean Zelinski Mule bass 5. That headstock had a love affair with gravity. Almost impossible to play sitting down and a bit less awful standing with a 4" leather strap.
Just because someone has not experienced neck dive on their instruments does not mean that it does not exist. Plenty of good examples.
Of course now I have a Warwick Corvette and and Fbass, so I'm out of the neck dive game.