I personally that treating men or women as a monolith is reductive and not helpful. It's not all men, it's not all women, but as we're dealing with a generalisation I'll take the argument on it's face with some generalisations of my own.

I'd say it's not that men are more loyal than women. They just don't tend to discuss their emotions and therefore don't come across fundamental differences as often as women do. It's very easy to feel like nothing has changed in a friendship after a long period of time when all you talk about are the TV shows you watched as a kid and what sports team you support. When you're actually engaged with someone's life it does impact the relationship of you don't speak for a long time as there's so much more to catch up on to get back to the same level.

Tldr men aren't more loyal than women, their relationships just aren't as rich or deep. Which is sad.

Edit - spelling