But it's better from many angles that they are. Discoverability alone. Consistency of instance level rules. Theme.
It just makes sense on some level that sports communities would be on a sports-focused website, and such a website is where people whose primary interest is in discussing sports would have their accounts. From there, they can follow other topics they're interested in, but their primary focus is still on, I don't know, basketball or whatever.
Same for cars. Some of the most active forums on the internet are car ownership forums. If you could access CivicForums from IoniqForums, then it would make sense to do so. Much more sense than finding people discussing Hondas on lemmy.world and Hyundais on sh.itjust.works.
Just because you don't give a shit where these discussions are taking place, doesn't mean it makes sense for people to just shit them out anywhere.
The onboarding process should be happening after this point. People shouldn't be going "I want to join Lemmy!", because that's kind of a non-sensical statement. Lemmy is a website engine. They should be going "I want to join awesomewebsite.com. Oh, and look, I can see stuff from anotheraweseomewebsite.net, too! That's so cool!"
If the website itself cannot provide a compelling reason as to why someone should sign up for it, then why should they sign up for it? So that it can be a dumb-terminal for some other website?