Why can't a user import keys to the keypool for a blank wallet? I am creating a blank wallet with the intention of adding keys to the keypool but get an error keys can only be imported to the keypool when private keys are disabled

Why? It would be beneficial for users to be able to import their own private keys and get full functionality of the wallet with those keys. Is there any PR for this?
When you create the wallet that you are importing your keys into, presumably with something like bitcoin-cli -rpcwallet='mywallet' importmulti, it needs to be created in watchonly mode, meaning with private keys disabled. To do that, create the wallet with bitcoin-cli createwallet 'mywallet' true, where true is the argument for disable_private_keys, creating a wallet for watch-only (public) keys.