That "something" is at least the entire userspace, so any attempt at doing so ends up being UX-equivalent to a full restart - while having a decent chance of leaving unintended trace data lying around in memory.
A full restart guarantees that everything will be wiped.
It’s not about data being wiped. It’s that neither Android nor iOS has
fully encrypted storage after you reboot and enter your credentials - biometric or passcodes.
Restart - simple with known and predictable effects, data no longer accessible, all secrets flushed no matter where they were or cached.
Turn off disk encryption, suspend all running services, overwrite all secrets in the O/S wherever they are, and then restore all that on entering password. Probably can't do anything about secrets cached by actual apps.
Complex, hard to maintain and probably buggy.
Why not flush something properly in the RAM instead to wipe the "cached" secrets?
A full restart feels like an overkill.