It’s lonely being an adult
I’m sorry your feeling lonely bro, if you have any interests consider joining a local group to make friends/connections. As for the question… eh there’s probably a varietybif reasons.
Outside of work most adults don’t do a lot, so being available and present in group activities is important for your social life. Most men socialize through shared activities rather than just talking, so if you’re a man make sure you socialize with people through shared activities. Some objective you accomplish together. There are clubs for that volunteer organizations many things
Obviously a lot of people here disagree. But I think for most people the answer is unequivocally yes.
I guess if you think the bare minimum for friendship is someone you say “Hi” to at the water cooler every once in a while then no, you can have lots of friends aside from your partner. But for many people your friendship/relationship with your partner consumes most of your time (outside of work). Add on to that kids, and that’s a wrap.
For most people that replaces the majority of the strong friendships that might have had when they were younger. Sure maybe they still have a couple of friends, hence the “less”.
Actually now that I wrote that I realize the answer is no. People consolidate their friendships partly due to maturity, but also in large part because they find friendship/companionship in their partner. People don’t get partners because they have less friends, they have less friends in part because they get a partner.