Because the decentralised ACL implementation (state resolution) relies on power levels as a simple comparator for determining merge conflict resolution. State resolution is the academically novel bit of Matrix and hard to get right, and while we can swap it out for role based ACLs in future, this is orthogonal to Spaces. Meanwhile it honestly hasn’t come up as a requirement, even from enterprisey folks, given we push power levels as hard as possible.

https://matrix.org/docs/guides/implementing-stateres and https://matrix.org/blog/2020/06/16/matrix-decomposition-an-i... give more info on how state res works and why you wouldn’t rush to jump to role based ACLs.

It’s nothing whatsoever to do with IRC or chanserv style behaviour, beyond coincidence.