A lot of theories about special interests by neurotypical experts concentrates on their use as a calming mechanism, something predictable and controllable that we can lose ourselves in as a means of dealing with stress and the weird, unpredictable world we live in.
Personally, I think there's more to it than that.
I do love my obsessions, but I lose myself in them because I love them, I don't love them because I can lose myself in them. If it was just about losing yourself, it wouldn't explain why some things press my obsession buttons while other very similar ones don't.
Here's one of mine: Dangermouse.
I can talk about this for as long as you'd like. Probably much longer than you'd like. In fact I could probably talk specifically about this episode for the entire duration of the video.
But Count Duckula, another cartoon by the same creators, in the same style, featuring some of the same voice talent, that started as a spin-off of Dangermouse? Nope. I don't care for it at all.
It's a bit like how just because you fancy someone it doesn't mean you'll also fancy their brother just because they have the same genetics and/or upbringing.
There's actually more to that analogy than it first appears. Some research shows that the brain chemicals seratonin and oxytocin appears to behave differently in autistic people, so some of us* may get less chemically-generated happiness from social encounters than one would expect. I've also read - although now that I want to link to it, I can't find the blasted thing - that we may in fact get that happy-lovely-brain-chemical goodness from our special interests as well as for people. So if that's the case, the love analogy is pretty apt, if our special interests do to our brains what lovely schmoopy times to do neurotypical brains.
*I say 'some of us' because this 'cares more about things than people' thing has never been true for me: I care a great deal, I just express it differently and often struggle to express it at all. I don't think it's at all fair or accurate to say that caring more for things than people is a standard autistic thing. I also think there are plenty of neurotypical people who care much more about things than people!