Because a rifle's reaction to recoil and torque is completely different than a handgun. A handgun's barrel rises much faster and is affected by more torque than a heavy rifle and the bullet may not clear the barrel in time to leave the muzzle before it starts climbing. You could clamp a handgun tightly in a fixed rest but it will not print to the same POI as when handheld. It's not a case of "inaccuracy" - it's about where you set the sights for shooting off hand. For informal testing I just use an old backpack stuffed with old clothes or an old Carhartt jacket I keep in the truck.
If the question is testing accuracy - it doesn't matter what you do as long as you do it EXACTLY the same every time to mirror the EXACT reaction of the firearm. The issue is try to keep POA/POI the same rather on a rest or not. A whole lot of Physics occur in a very short time period in a handgun.
I can relate to this personally with a scoped 44 magnum. I kept on going back forth on a bag to handheld and could not get the damn thing to stay at a POA/POI. Given the weight of the gun, after awhile, I was resting the gun on the bag even though I knew better.
Once I rested only my forearms on the bag; voila! - consistent POA/POI.
This is why we freefloat and/or bed rifles to create consistent and repeatable physics in the firearm.
Doesn't mean a rest isn't valuable. I like to know what a handgun is capable of and then chase that accuracy without the aid of rest.