> Why couldn't an encryption scheme be used that, for decryption, requires either a) the user's private key (a usual), or b) some sort of combination of private keys of all the relevant government entities required to authorize accessing the user's data?

It maybe be technically possible to make such a scheme, but due to the unitary executive, keys split among N government agencies have the same essential practical security as 1 key held by one counterparty, and one counterparty that has no particular interest in the security of your data. So this fails the “preserves security” aspect for the same reason that any system that enables government access must.