We simply have far bigger priorities than dedicated the enormous resources to having a whole separate set of releases with userdebug features and an on-device root terminal which requires a special boot mode and isn't persistent. It's theoretically possible to do it in a way that doesn't throw away a lot of security, but isn't being done by anyone right now. Resigning the OS doesn't avoid the major security loss from having it integrated in the way these projects are doing. It gives the false perception of having the verified boot and other OS isolation security intact despite totally ruining verified boot by trusting persistent state and hurting OS security a lot beyond that. It is possible to do it without those problems by having it limited to the user using it, but that means preventing apps hijacking it in any way such as accessibility services.