Modern chainsaws have a chain brake that stops the chain instantly when the saw kicks and your wrist contacts the handle/trigger. It takes very little pressure to trigger. If you test the chain brake every time you get the saw out, and hold the saw correctly there is very little chance of being hit by a moving chain, unless doing something out of the ordinary like carving.
An impact design would make the saw basically unuseable. How long do you hold the trigger open on an impact wrench in one go normally? 30 secs to a minute. Now get a bolt and weld it to the shaft and try to get it off with an impact wrench. How long can you hold the trigger before you need a break from the vibration?
Mechanics tools arent designed for continuous use. You put them down while positioning parts, and doing other tasks.
Chainsaws are designed for pretty much continious use. You don't cut a piece of wood, turn the saw off, stack the piece of wood and then restart the saw. You continue to cut until the chain needs a sharpen, fuel or chain oil runs out or you run out of wood. With a good crew and a couple of saws you can cut non stop all day if you want.
As others have said, you want the saw to run as smooth as possible. Its easier on the operator, easier on the machine, safer and faster.