Friends ghosting me after they have a GF and I don't understand why?

I've noticed this has become a pattern in my life, and it's starting to hurt me. I'll have a friend I've known for years, we'll talk regularly, hang out, joke around, and everything seems completely normal. Then they get a girlfriend, and within a few weeks or months they slowly disappear. Sometimes they stop replying, other times they just block me out of nowhere without any explanation.

I genuinely don't understand why this keeps happening. I never flirt with their girlfriends, I don't disrespect their relationships, and I don't expect to be their number one priority. I know relationships take time and effort, and I completely respect that.

It's happened enough times now that I'm starting to wonder if I'm doing something wrong, but I honestly can't figure out what it is. I just feel really lonely and upset because these were people I considered close friends. What do I do?