The first simply had another passion - travel. Work was just a way to pay for that. Eventually went to work for an agency, been there a long time and AFAICT couldn't be happier (despite being less well off materially). I've known a couple of others who fit this pattern. One left the industry to raise goats and make cheese instead.
Multiple have left to become full time parents. I hope they don't regret it, since this group includes my wife. ;)
Several others have left the industry but have not necessarily left tech. Some do light consulting. One's writing a book. Most are working on long-deferred personal tech projects.
I just about joined this third group before my savings took a 15% hit, so I might as well say why. I'm tired. I'm tired of the artificial deadlines, and processes that slow people down more than they improve quality, and the omnipresence of coworkers who exhibit every kind of bad engineering or interpersonal behavior (even though others are awesome). I want to enjoy making things again, and the moments when I can do that within the industry seem all too fleeting. Even the best of my dozen jobs stopped being fun, or just stopped. The thought of going through a modern tech interview process yet again so that I can do all the rest again just fills me with dread.