Because some players build the PC they want. What about more of an edge case, like a Navy SEAL who served on a submarine and has Nuclear Science.Why would the player choose a skill the PC doesn't need? If I'm playing a hard boiled 1970s era cop, I'm not going to take Physician as a skill because it's not going to be all that useful.
Also, GURPS does a lot of heavy lifting with templates for characters. Some of these are very through but might contain extraneous skills because of it.