The phone's materials are too weak to cause much deformation of the bullet. The phone itself is designed to dampen and distribute the force of small impacts (to avoid internal damage). So once the phone's materials reach their elastic limit, the remaining force is then concentrated in a small area, which causes most of the phone at the point of impact to shatter and disperse.
This detritus ends up absorbing much of the transferred kinetic energy (as can be seen when it explodes out the rear at high speed) which means less of this energy is being transferred to the entire object. Also note how much the back of the phone becomes deformed; while the screen and internal components easily shatter, the metal back distributes some of the bullet's energy tangentially from the point of impact. Finally, the fact that the bullet is traveling faster than the speed of sound will cause the air in front to act as a sort of "buffer" prior to and during the impact. This decreases the amount of physical contact between the phone and bullet, lowering the friction between the two objects, thus again decreasing the energy transferred into the phone. These events all infer that a good deal of energy is being expended on forces that don't serve to displace the phone's remaining mass.
The bullet and the phone also have different masses. While the phone weighs roughly 112 grams, a 50cal sniper round might weigh between 39-48 grams depending on its type. Additionally, the stand the phone is attached to weighs an unknown amount. So the phone's much larger inertia needs to be overcome by a bullet whose forces are being largely redirected and dissipated.