Joe, your point and question is a good one. I can definitely see why some people wouldn't want to leave for retirement.
While I am nowhere near retirement and, frankly, I believe my wife and I have found the location we want to live out our days, I would have no problems moving for retirement if it made financial sense and the retirement location checked the necessary boxes (good weather, good golf, decent shopping option and restaurants for my wife, etc.). Until our last move, I always assumed I would move to some nice retirement location (or, have a winter location in a place like Phoenix and a summer one in the northwest or something).
As to the friends point, my perspective is:
1. I am one of those people that is very good at making what I'll call activity-centered friends. In other words, I'm fortunate enough to be able to "fit in" with most groups and make friends with just about anybody with a common interest - usually revolving around a sport or a game. For example, I have been an avid baseball/softball player and golfer in my adulthood. Every time I've moved I've been able to pretty quickly find a team or a good golf group in a short period of time. Currently, I love my golf club and my fellow members but I am pretty confident if I moved I could find a very similarly enjoyable club/group. So, that social itch for me is pretty easily scratched.
2. Somewhat in harmony with the above, I am not the type of person who needs close friends. This is going to sound cheesy, but my wife is my best friend and is really all I need for that "close" emotional connection. Accordingly, with having lots of arms-length friends associated with certain activities, I'm honestly good about leaving communities.
3. Technology and means to travel really make it easy to keep those connections. Obviously, this forum is a great example of technology permitting me to have a genuinely positive online community whereby I can talk about ideas with other humans. Similarly, with text messages, Zoom video chat, etc. it's easy to stay connected with friends from afar. Heck, fantasy football is a great tool for keeping in regular contact with friends from high school and college as well. Finally, I am fortunate enough to have the financial means to travel to see friends from prior places pretty often. So, still playing in softball tournaments with a former team based three hours away is pretty easy and I'm still close to my old golf groups even though I moved away because we are able to plan and afford a few golf trips per year to keep that connection going. I imagine this would only be easier in retirement with the additional free time.