Because they can?
Because they want to learn?
Because they haven't found an instance they liked?
Because they want to create an instance for some specific interest and existing instances aren't the right fit?
Because they want a private instance for their family/friends?
Because they want to use it a bit like a client, deciding which instances they interface with?
Because virtually every instance is simply someone hosting their own and letting random strangers use it.
because they want to be in charge instead of being at the mercy of someone else?
like I just got a 3-day ban from some instance or community because something I said made the owner mad, and I imagine if I ran my own instance and community I wouldn't have been banned 😄
I guess you could consider it condescending but you did not ask like how but why. Why ride a bike. Why own a dog. why get out of bed. Why questions are something that really is up to the individual.