In the 1990s, it was not uncommon for long-term parked a/c to be re-positioned, to free up stands for impending div-arrivals. I seem to remember 3 in the engine-run bay on several occasions.
Who's going to pay for these aircraft to be towed around willy nilly on the off chance the airport gets a divert?
If I was airline A, I'd be pretty pissed off paying £400 to have my aircraft towed into the test bay and back to accept a divert from rival company B.
It just isn't practical