Nothing is stopping them from doing it, which is why when the cops get a report of someone pointing a gun at people they have no choice but to assume it’s real (unless it’s obviously shaped like a super soaker). If the person training their gun-shaped object on passersby points the gun at the cops they usually get shot because the cops aren’t willing to bet their life that it’s not some asshole who put orange acrylic paint on the tip of a real gun.
There are some hobbyists that paint their guns to look like a Nerf gun or a Nintendo gun controller. The general consensus is “it would be stupid to take this out anywhere other than the range”. This is much less common than the opposite: toy or pellet guns that look very realistic.