This topic has come up here many times before, including in this thread:
"Do undercover cops offer outcalls?"
I think the consensus of opinion is that female cops posing as hookers don't often do outcalls because of the danger stemming from not being able to control the environment like cops can when they book the room. I think it's reasonable to assume the police don't want to send an officer into an environment not knowing if there may be weapons, multiple parties, or other dangers in the outcall. Further, the UC officer may not be able to communicate with the officers outside. Also, if a UC did outcalls, there'd be no assurance the john's playpen would be located in the city where the UC works.
Regarding incall stings, a common m.o. for cops is for the girl to be near the bed and cops with guns drawn burst out of the bathroom. Sometimes the police will have audio- and video-recording equipment in the room. Obviously, you can't do that or otherwise control the environment on an outcall.