Isn't that aspect of Keycloak a carryover from the days when one VM held one instance of an application? These days containers are cheap and you can just spin up each "realm" in another container.
I do run Keycloak in a container but I'm pretty sure spinning up a new instance for every realm would be more resource intensive than using multiple realms in the same instance.
It's just a question of use case at the end of the day. In my use case I only need this for small internal tools so it's easier to just spin up one instance for me.