The problem is, that if you have an account at InstanceX, and you are browsing the website of InstanceY, InstanceY doesn’t know where your account is. I think to solve this problem, there would need to be a browser extension that knows where your account is (you configure it) and that converts Lemmy addresses to point to your instance.
Additionally I think kbin isn’t open to signups anymore from what people are saying.
kbin accepts signups, but kbin.social is currently struggling, so one might want to choose another instance such as fedia.io.
Lemmy.world was just migrated to a dedicated server: https://lemmy.world/post/75556