Why? Many possible reasons.
If you're talking about why as in -- how autistics are hardwired for it, well I'm not sure that myself.
It could be it just it -- like stimming and for all it's purpose, the focus to stimulate and drown out anything else.
It's mesmerizing, addicting... The deeper, the harder it is to pull away. Sometimes whether you like it or not.
It can be a sanctuary to cave in, or a bridge to go and connect with -- against or towards the uncertainties and unpredictabilities of the world. It charges you well and put you on stay, or propels you further.
Only much more complex than stimming, tend to involve more than just the senses and involving different scopes or dimensions, and more explicit learning involved.
ANYTHING can be a special interest!
Living, nonliving. Conventionally sensational or obscure, taboo or trendy, academic or not, fact or fiction, useless or practical -- anything really.
The scope of topic, the connections or relevance, how long it lasts, how intense, and how it is conveyed is entirely up to the individual.
In my case, I could have some special interests and many interests.
My primary and long term special interest is crafting.
Though I don't recite special interests, rarely ever know it's terms than most human would, or even less so talk about it. I'd watch them just to figure more 'hints' instead of watching them over and over. I mainly do it and think about it.