Because if I wanted that just stay with Twitter, and that's the attitude of a lot of us.
I run my own instance. I can't do that on Twitter. If I get tired of it, I can move to another instance and most of my followers will migrate automatically (not all, yet, because not all ActivityPub software supports the move mechanism, but when I moved to my personal instance 80%+ already moved automatically, and it'll go up).
Instances can be blacklisted on Mastodon, but have far less of an impact unless you're so particularly abuse that a large portion of the network blocks you.
If you don't provide means to do robust blocking and moderation that will be another reason for people to stay away. Mastodon has these "issues" because it is what users want. Anyone can say exactly what they want, but anyone can also choose to block you, or your instance, if they don't want to listen. Gab is an example that is on Mastodon and is widely blocked because most people don't want to interact with them, and that's our right, but they're still free to talk to those who want to deal with them.
