I use my (unprotected) phones indoors and outdoors all the time (including taking lots of photos while hiking, for example), and I've never scratched a screen. So it's not impossible to avoid.Impossible if you take the phone outside. I never use a screen protector and I have tried everything not to get scratches. As soon as the phone goes into a pocket it's going to have scratches.
The only time I protect my phone is in a super-sandy environment like a beach. Then, I keep the phone sealed in a plastic bag to avoid scratches from the sand. If I were in that sort of environment often, I would probably get a screen protector.