My guess is that I’m not interested in people at all, and so are my remaining friends. From time to time you meet a person who has a potential to be friends with, but all the potential activities you’ve already seen and got bored of. I don’t get drunk, high, do not do crazy things or sharp discussions anymore. Common interests are just that - common interests. What makes you friends with someone is not just spending time together, but doing something you’ll never do with random people (though few people think they are your friends because they spent some time with you). Now most of ‘still friends’ that I have simply offload their problems to someone’s ears when we meet, or endlessly discuss material matters like property prices, car options and female parameters.
I don’t think you can make true friends in 30s unless you’re taking part in a war-like activity.