Sharpshooters are typically deployed in pairs. One acts as the spotter, who does the calculations, and one is the triggerman, who is responsible for adjusting the rifle and taking the shot. Trained marksmen also do not usually aim for the head if the center of mass is exposed.

For anyone who doesn't realize how far 150 yards is and how difficult a shot is at a distance, a target that would be 6ft tall at 1 yard shrinks to just under half an inch (31/64ths) in size to the naked eye at 150 yards, and that's the size of the target from head to toe, not just the head.

The human head, meanwhile, is about 9 inches from chin to crown for someone 6ft tall, which reduces to less than 0.05 inches at 150 yards, assuming you have no magnification. An untrained shooter would probably need at least 10x magnification to make the shot, and that's assuming they account for wind and have their scope properly zero'd in for that distance.

EDIT: To make one thing clear, I think we are all collectively very lucky that the shooter did not account for wind direction and speed, because that is the most likely reason that Trump wasn't killed.