a border run requires wasting a whole day sitting in airports and on planes and is more expensive. An extension is cheap and only takes 1 or 2 hours at immigration if the documents immigration want are supplied
The extension is easy. It's the same documents you used for your application with updated dates, plus proof of where you are staying.
An extension is faster and cheaper.
Why would you want to waste a whole day sitting in Airports and planes and spending more money when all you have to do is spend an hour or two in the evening printing out documents and then the next day going to the immigration office for maybe a couple of hours....