Because it's a dumb thing to be doing.
You probably don't realise that many aircraft don't have ADSB-out and your app won't be warning you about them.
And the planes that don't have ADSB are mostly smaller planes which are the ones most likely...
Not ALL aircraft have to send out that signal and what if that unit failed one day and was not broadcasting and you went to fly over the airport?
You are correct that if you fly over the centre of the runway there will be no aircraft right above...
I'd go further and say STUPID. I'm sure you'd feel quite good sitting in court one day after you flew across a runway and caused a GA aircraft to crash. Sitting there as the cool drone pilot, looking at his wife and small children, whose lives...
"I'd have to be real careful crossing the runway"
Why would you fly anywhere near a runway of even a small airport? At least it will let you takeoff, then fly away from the airport. Anything else is just irresponsible.