In my case marriage was the great motivator for leaving frendiship and not having new friends. My old friends who are single now, still have good friendship, even after 30. I guess my best friend is my wife now.
Also when spouses go and spend time with the other spouse's friends, its challenging to simultaneously catch-up with their old friends while also trying to include their new spouse in the conversation.
Some married couples work through this but others just privately and repeatedly criticize each other's friends so eventually they are left with only one friend -- their spouse.
You can't just be friends with one of them. You can't just go and hang out with one person without their spouse coming along. If you tell them their spouse isn't invited, they'll get offended. If you don't tell them, you can expect the spouse to tag along... and if their spouse gets tired and wants to leave early, they're both just going to cut out right then and there (I had one friend who would just up and leave the bar with his wife whenever she got tired without even giving me a chance to finish my drink and tab out so we could leave together... every time, it felt like a slap in the face).
Sometimes, even if I like their spouse, I just want to hang out with a friend one-on-one. And then there are people who I like but whose spouses I can't stand.
Seriously though, marriage doesn’t mean you can’t have friends, keep friends or make new ones. It’s a twisted sort of relationship that requires you to give up your friends or only spend time with your wife.
My wife is my best friend but my 2 out my 3 kayak buddies are women, I spend that day with friends helping them build motors, I go on road trips with friends, I go off for mountain biking weekend and ski trips with my friends, etc.
I will say, my friends who have kids do tend to drop off the radar but you can’t blame that on marriage.