Hobbies are almost by definition not mandatory activities. Earning money to eat and pay the mortgage is not really optional. Making the hobby activity mandatory will make it feel a lot more like work and a lot less like a "fun" activity. It might not be as bad as working in an Amazon warehouse, but it'll be a job all the same.

Worst case, if you get burned out on the new job you will not only no longer have a job but also won't enjoy your hobby anymore.

My experience as someone who loves drawing and does often delightedly remind herself that her day job is sitting around a park drawing shit:

basically, yes, all of these reasons apply.

I'm good at drawing, not at selling myself; my work sells itself to a certain degree but that only gets you so far. Not everything I'm paid to draw is a thing I love to draw, either. And sometimes even a thing I love to draw becomes less fun when it has to meet someone else's approval; going back and forth with a client is a very different feeling than "I start drawing and I fix things I don't like until I'm happy with the piece".

Deadlines suck and I mostly arrange my work so as not to have any. Yes, this is a luxury. It probably means less money but I make enough to get by.

Most hobbies doesn't have sales, marketing, taxes, regulatory mandated recordkeeping, deadlines, people yelling at you, or "regular business hours". Businesses have most if not all of these.

>Or because of the pressure of delivering the result?

It's mainly this. It's not a hobby when the customer needs their product delivered by Thursday, not is it a hobby where you need to maintain a certain output to pay the bills.

Sales and marketing are just a thing that you get used to.