The thing is, there has been a few fatal crashes lately that was the result in turn back, crash, die scenarios... guess what, all from high time experienced pilots. As Motz said when something such as the engine stopping happens, your ability drops significantly! I had an engine failure about two years ago on take off, was only about 10ft off the ground and still had runway in front so just put it down and came to a screeching halt just in time! I couldn't really recall what I had even done during that stoppage period, once on the ground I noticed I'd pulled throttle to idle and landed the aircraft and applied brake. The point I'm trying to make is, when something happens your head turns to glue, so it needs to be totally second nature to do the things you need to do.
An engine failure at height you have a minute or two to gather your thoughts, near the ground you have seconds.
Turning back will work when you're at 100% as it's just a low flying manoeuvre... chuck a mushed brain into the party and you are pretty much asking for trouble.