Making friends as an adult is extremely hard.
People are set in their ways. That means that if you are outside the norm, whatever that means for where you are, it will be hard to meet new people, and get along with them.
People in their 50s seem to be very inflexible in their beliefs, and not will to agree to disagree. So you can either not be friends, or not be yourself. Neither are very good choices. So ultimately it's easier to just be alone.
Throw into that nonsense all the bullshit that comes from meeting people of the opposite sex. Constant comments from everyone, it sucks. Most of the new people I have met, and made friends with, are women. Why, I have no idea. It could be that they don't spend all their time and energy trying to prove they have the biggest dick. I don't know. It could be that women just appeal to my more sensitive side.
Recently a friend asked how I am what I am. By all rights I should be part of the problem. Middle aged CIS white male. But I am not, not even close. I know exactly how this came about. I distinctly remember the aids crisis during the 80s. I heard stories of couples that had been together for decades could not be together in the hospital. How people had let their partners die alone. Because they weren't married! WTF. What does being married have to do with caring, loving for someone? Those stories were the most heart breaking, inhuman thing I had ever heard. They changed me forever.
Anyway, I have some new friends, most of which are women. Despite the comments I heard from EVERYONE, there is nothing beyond friendship. I don't know why the base assumption is that there is something else behind it all. I value their friendship more than anything.