Because the door being open makes it possible for opportunistic thieves and even kids to steal something. If the police knocks on it, it's actually better to open it. Otherwise they will still get in, but you will also not have door after that.
With phone interception, I can't imagine any other actor being sophisticated enough to bother with setting up the stringray thingy. Maybe something very targeted to get somebody very special (having a hot wallet with 20 bitcoins and going around the city with it comes to mind), but I would still expect the simplier methods there too.
Add: Even with the normal HTTP traffic, mitming was way more common and more practically exploitable back in the day, just by setting up a rogue wifi AP and fishing for passowrds. I'm not sure it was ever a thing with stringrays when non-government actors did something with them.
