Strange question. Hockey fans don't worry about this, it just is.
Goalies are strange, not many people will stand guarding a net with frozen, hard rubber disks being fired at you at 90mph. At the upper levels, goalie equipment is custom made to fit the goalies stature for the best protective coverage. So they do not keep a set for a position player to be an emergency backup.
Most kids have specialized as goalies by age 12, so an amateur will likely be better than a position player.
Seldom is an emgency goaltender required. Rosters are capped at 23. Seldom will a team use a spot for a third goaltender. If they do, it is because they have 3 they want to keep, but because of contract status, they don't want to expose one to waivers when being sent down to the minors.
