If not a password, what alternative option are you going to use to decrypt your disk/data?
Almost any other option would likely require some additional complex software infrastructure just to use such a feature.
In my opinion the iPhone is not a very good model of general computing devices, because it’s effectively a closed platform that is locked down pretty tight. Never mind that it’s operating system, APIs etc are developed and maintained solely by a corporate entity which can enforce things being done to a standard and tested thoroughly. You have to jailbreak it to install any software Apple hasn’t reviewed and approved.