I think the unsaid part is just time spent together- when you’re a kid it’s easy to have dozens of hours a week to hang out and bond. As you age, there’s other time commitments - kids, spouse, family, maintaining a house, etc. In order to have that emotional investment you need to get past the awkward first stages of friendship.
I think a lot of people lose/drop their hobbies, or the things that let them bond and meet other people. It’s hard to say “I dropped football and now I lost 50% of male conversation” without more info. If all your friends are only bonding over football, yeah. So find other things to do! There’s a million of them, and people are always passionate about their own interests. Find people with similar interests.
The author also mentions “it feels like they’re always just someone’s partner” and that’s very telling. Are the only men you’re engaging with those who are partners of your own spouse? Well no shit you’re not feeling like you have friends. I like my wife’s friends partners, but they’re firmly in the acquaintance category.