It’s a snapcraft limitation - no one on the team was strongly opinionated on migrating this feature into core24 and it requires development work to support in snapcraft due to the craft-application integration and with launchpad for remote builds, so it wasn’t prioritized.
That being said, build-for: all could still be added in the future. Can you give us some use cases where building a separate snap for each architecture doesn’t work (or is inconvenient) and links to the snapcraft.yamls where you are using it?