I think installing spyware on someone’s device is two or three steps more drastic a measure than a simple search, which is about the extent of what a court order can authorize police to do right now. It feels conceptually close to tampering with evidence present at a (possible) crime scene. To add to this, spyware is not the same thing as installing a physical listening device in someone’s home. It requires far-reaching permissions on a system, and can influence lots of other software on the same system. You’d have to have an extreme level of confidence that this won’t lead to accidental or intentional planting of incriminating material. And, in my opinion that sort of load-bearing trust is not really something law enforcement has earned in the general case.