When you're young (<20), you don't have many responsibilities or long term goals. But as soon as you get out of college, you have several multi-decade goals to work towards:
1) asset accumulation
2) lifestyle development
3) family raising
From a pure risk management point of view, assets, lifstyle, and family are much more stable in terms of extracting value from.
Friends, from a value added point of view, are a risky investment for a couple of reasons. They require time and commitment to make and get value out of, but have a high chance of no return. If you invest too much in friends, you're pretty much guaranteed that something out of your control breaks the friendship by 5-10 years, either moving away, diverging interests, or the most likely, the friend decides to invest time in the assets, lifestyle, and family instead of your friendship.
There's a vicious circle (opposite of virtuous cycle) in this. As a society, if more people focus on family, assets, and lifestyle because they are less risky, there's less people willing to commit to being a friend available for those that want to focus on friendships, and eventually friendship by the age of 30 become extinct except in rare cases where special environments sustain them.
To summarize, it's a systemic issue where even if you wanted to make friends, it's not worth it as an adult to do so.