Just wondering, are you an only child?
How to socialize with others starts early, if your parents didn't mix much maybe you never learnt to make friends?
My mother didn't like company, never let anyone in the house I was a lonely, shy kid. As a teen I longed for mates, to be popular it was hard. To make friends didn't come naturally to me.

I don't really agree with the outside hobby thing…if you really want to excel, your career needs to be your hobby. Basically, it needs to be something you're that passionate about. At some point, after you've put in all the work, you'll have some extra time to maybe explore other things if you so choose.

I'm in a similar position. I didn't socialize during my masters and I accomplished so much more than if I had. I'm now working on my doctorate and have a boyfriend now. I sometimes wonder if I'd get more work done if I didn't, but I have experienced many things and learned many things that would help me in my future life/career so ultimately it's worth it. Even if I'm now only producing the amount of work of one person instead of several. The way I met him and connected with him is because he's also getting his doctorate in the same career field. It would probably be the most beneficial to make acquaintances (and hopefully friends) within your peer group/career field. Now I'm not 100% sure how to go about that because the connections I've made were pure luck and based off the persistence of the other person. It's apparently difficult to get close to me. Perhaps it's the same with you.

And I didn't connect with many people in undergrad either. I really doubt everyone actually has a social life. They just like to make it look that way. And while they're wasting their time, you're getting your work done.

Or perhaps go to grad school someday if it would be useful for your career. You'll finally be surrounded by peers who are your intellectual equals and it might be easier to connect (when you have the time). It's really nice.

It seems to me that you feel something is not right about your life. If that is what you feel you are probably right. I suggest you use your obvious intelligence and analytical skills ot look into this. Assuring yourself could be crucial to your future. Universities have counsellors - speak to them perhaps?