A lot of instances have "themes" because they are a way to gather likeminded users. There are also general instances without as all-encompassing themes, and of course you can use say a tech-oriented instance to post your own art if that is permitted by it's rules.

You sign up for the moderation you want, and if it or you changes you can jump ship while being able to retain connections (trust me, I've seen several users having migrated instances).

What's great about the Fediverse is that you not only sign up for the network, but the point you start from inside it, and the moderation you want. On a fully decentralized platform you'd either be subject to some form of easily gameable group-moderation or be left to the task of filtering content all by yourself.

I don't care that I might be missing out on some maybe nice person on a instance defederated on my home instance. I care that I feel that I belong on my home instance and my feeds are full of people whose community I can feel like I belong to as well. One that has been very easy to grow organically without an algorithm telling me what I want.