Because when - not if, when - that government-owned master key gets out
I don't think anybody is seriously proposing a single master key. The value of something like that would be much lower than the potential costs for exactly the reasons you laid out.
Keys are essentially free. There's really no good reason to make just one and use it for everything. Instead, they should be generated as needed and should have a realistic expiration date. Better yet, the keys should probably be managed by Apple and Google entirely. They are already doing key management, seem to be very good at it, and there's no good reason to take over that role.
Decryption keys are fixed, factual things that can't be restricted to "only the people who we think should have the authority to use them".
Is there any kind of surveillance done by law enforcement that is guaranteed to never be abused? I don't know of any and I don't suspect law makers think digital surveillance should be held to a higher standard.