It can be as simple as two clicks. All that's needed is Bitcoin wallet app devs to implement the needed functionality.
Why not use something like OpenPGP that was designed for it? Why can't users have web-site login sub-keys they access with two clicks?
Edit: thought on one possible reason: Bitcoin is standardized on
one signature algorithm.
Because not every Bitcoin user has some pgp software installed, but all of them use a Bitcoin wallet already.