Probably because the FiA is full of old boys, and it's tradition, innit.
There was a time when drivers had a varying number for every race.
Here is year of Dan Gurney numbers, I think it was to do with what entrant number they were. I like the idea of different numbers for each race, as I like privateers and cigarette advertising.
Mexico 3
USA 11
Italy 8
Canada 10
Germany 9
Britain 9
France 9
Belgium 36
Netherlands 15
Monaco 23
South Africa 9