Each instance admin decides which servers to block for themselves. If you visit the info pages of some systems they will list blocked systems, and there are a lot of them.
There are some very unsavoury communities out there. Blocking usually revolves around how effective moderation is.
As an example you can see a list of servers blocked by mastodonapp.uk on the About page.
I’m guessing you haven’t been on the #Fediverse very long so not picked up on the ethos of most of the folk who run the various instances.
Most are very protective of what they have created as a community and are definitely not in it for the money. Some are vehemently anti-capitalist.
There are many ways to get rich. Running an instance is not one of them.
Oliphant maintains a minimum block list that most systems take as a starter list.