You're definitely not the only one, people get more socially isolated as they get older, mostly by choice.
Think about what "hanging out with your friends" meant when you were young. How much time did you spend just sitting there doing mostly nothing in the company of your friends and you guys just shot the shit and and fucked off?
You don't want to do that anymore as an adult, right? You got shit to do, you want your entertainment productive and timely and not a waste of your time. You can't go sit at your boy's house and wait for him to get ready, wait for friends, someone forgot their wallet, have to run to their house... by the time you get to the "out" place, it's 11:30pm and you're thinking about shit you have to do tomorrow.
My $0.02 - if you want to feel like a kid again, act like a kid. Go do dumb shit. Ask your friends (or prospective friends) if they just want to hang out. Bring some drinks or some joints or whatever your poison is, and just hang out. No expectations.
But also
Hatorade
is exactly right - for men most friends are activity friends, like a car racing friend or a video game friend or a sport friend. Thats how you met, thats the stuff you do together. Thats fine too, but I'd make activity friends and then try to convert them into general-purpose friends.
Friendships as an adult take work, they take patience and forgiveness, and they take you not doing your #1 preferred activity all the time (and the friend too).
It's no wonder most people just talk to randos online, people suck and take work, and nobody feels like they have time to waste while they also doomscroll 6 hours a day.