We have SIM cards for 30 years to authenticate unique users to the network, but those same cards can't authenticate the network? No, this is entirely by choice and could have been trivially solved. They just forgot the "server certificate" part.
Also, why couldn't law enforcement simply coerce the cellular carriers to sign their stingray cert? It's been known to happen for SSL: https://arstechnica.com/information-technology/2010/03/govts...