In early Steam Deck showcase videos there were talks with Valve guys like Lawrence Yang, and IIRC they simply said that it is easier for them to build the system that way, not that they couldn’t continue using Debian.
I think the reason for that might be that Debian has pretty strict package and dependency policies and sometimes it’s not easy to put cutting edge solutions on top of the „stable” base, so they would end-up using unstable/sid anyway, which still isn’t ideal as there is some freezing happening every now and then. Also Debian packaging system feels quite dated and strict comparing to PKGBUILD format, and it’s simply easier to build custom packages, having single build instruction file is super convenient and unlike with Debian at times, replacing whatever core system packages without breaking half of the dependency tree is usually easily doable on Arch.