Hmm. I get other people to build my car, grow my food, generate my electricity, extract and refine my petrol, clean my water, I've rented cars, I get others to fly planes for me that I don't own, I use trains others drive and own.
I do everything for little to nothing in my life, and there's no reasonable default as to where the line is other than a cost/benefit comparison.
For my for many years owning a car was far more expensive than renting or getting taxis when needed. Owning a car absolutely would have been an expensive hobby, and the same is true for many in cities.
Having a personal server is exceptionally cheap. I had a VPS unnoticed recently I'd forgotten to cancel which cost about 10 dollars per year. That's about one minimum wage hour where I live. If you mean literally a personal server a raspberry pi can easily run a bunch of things and can cost about the same as a one off.
It's time, and upfront costs of software. If I want updates, and I do (security at least) I need some ongoing payments for those, and then I need to manage a machine. That management is better done by people other than me (as even if they earned the same as me they'd be faster and better) and they can manage more machines without a linear increase in their time.
So why self host? Sometimes it'll make sense, but the idea it should be the default to me doesn't hold. Little needs to be 100% in house, and sharing things can often be far more efficient. Software just happens to be incredibly easy to share.