IIRC main Fedora used to not do this until some update crashed people’s sessions including the update process which left their install in an unbootable state.
The ostree based versions like Silverblue avoid this by their updates not touching the running system and instead creating a new folder structure with the updates applied that will be booted into on next boot.
If I were to speculate, they are waiting for the NVK (Open source NVIDIA driver) to be more mature. So, they wouldn’t have to release two versions and wouldn’t depend on NVIDIA to update their driver to work with software the Steam Deck uses. I.E. Steam Deck uses gamescope for everything outside of Desktop mode. NVIDIA’s driver didn’t work with it until 2 months after the Steam Deck release. Even though it had existed for years prior.