They sell because it’s their retirement nest egg.

Most private practices are small groups, not individuals. It absolutely sucks to have to find a new doctor who’s a good fit for you, when you’ve been seeing the same person for years or even decades. Usually when your doctor retires you rely on them to make a recommendation for a replacement. Usually that’s another partner in their same practice. At the very least, you (historically) expect them to sell the practice to someone they think is a worthy successor. Patients are fairly sticky.

And many of these PE sales require the selling partner to stay on for at least a year or two to ease the patients through the transition and so the patients get used to the changes before the doctor actually retires. That way, sticking around after the doctor retires doesn’t feel like such an upheaval.