I sincerely wish that wasn't the case, but in addition to some allusions in this very thread as to why, it's pretty widely recognized:

https://www.theguardian.com/society/2015/nov/07/why-men-lose-friends-in-their-30s

https://www.vice.com/en_us/article/nz8k3q/five-ways-you-lose-your-friends-in-your-30s

http://www.executivestyle.com.au/why-do-millions-of-men-have-no-close-friends-gl3tu2

I was sincere about the insularity of it all. I was thinking about getting married once, and an old head told me if anything goes wrong in your house, people won't look at your wife - they'll look at you. In trying to be the head of household, stay employed and provide for the kids, keep the wife happy, etc. men can find themselves just digging in and "doing the work" day in and day out, neglecting what gives them fulfillment like hobbies and friendships. Women with the commonality of family and marriage tend to make and maintain their relationships well into adulthood based on that. For us, not so much. The first article I posted mentions that a lot of men, while even in relationships feel isolated.

On top of that, there's a lot more at play that causes male friendships to die. After marriage/family, they may feel they can't relate to their friends anymore (especially if they're single - and it reminds them of who they once were) and lean on their wives relationships more - the "I'm grown now." stunt. If they're beholden to their family, there may be jealousy if someone they started off with has more latitude financially, in life, whatever while they have to schedule carpool or take the long way home from work to get some alone time. More often than that, it's just ego. No one wants to break or be the first to reach out. "Oh, he'll be aiight. I'll hit him next week."...and then next week never comes.

Anecdotally, it's happened to me. My best friend for almost 30 years got married a couple years ago and then basically cut me off. I did my part to reach out and repair the relationship, but it wasn't reciprocated. It is what it is. People and circumstances change, but I think there's something to be said for being a stand up dude and having a community around you that you both gain from and you put into, who actually have seen your development. Maintaining close friendships is part of that.