Another factor I'd contribute is that you don't want to get too far away before doing your runups, finding an issue and now having to vacate a runway. A lot of places OCTA you'll find the spot you're parked in will just as likely be perfectly suitable for doing runups. Not to mention the possibility of exacerbating an issue you've just found by having to taxi back to parking.
Most places OCTA you won't find heaps of aircraft parked up, they'll be a single line away from buildings a lot of the time, so just do it on the spot. Why waste time giving all your calls, taxiing out, doing your checklists, getting to the runway, running up there and then finding a fouled plug that needs a few minutes to clear when it could have all been done before taxiing.
Honestly as others have pointed out, it comes down to common sense, do it when necessary, do it away from others as much as practically possible and reasonably necessary, watch your prop wash and IMHO as soon as practical.