Same reason ain't nobody gonna grind out hash collisions to enable covenant functionality.
It's a pain in the fucking ass.
Discussion
Doesn't seem more painful than having an OP code or new SIGHASH. If anything, there's a lot more logic that you have to wrap your head around with those than something much more straightforward, like having signed transactions ready to go.
Securely generating and destroying keys on a regular basis is a massive pain.
These types of protocols are more conducive to hot wallets / hsm backed infrastructure. It's really not conducive to using in conjunction with cold storage.
Have you seen the people who actually manage HSMs in the real world?