Why retire numbers when someone is still wearing it?

MLB should wait to retire numbers or ask players wearing that number to change before retiring the number.Why retire numbers when someone is still wearing it?
Numbers are normally retired by an individual team, not by MLB. Usually after a great player retires, his team will stop using that number unofficially, so there is usually not a problem when the number is eventually retired.





The only number retired by MLB was number 42 for Jackie Robinson. This is the only number retired for all teams. Current players at that time with number 42 were allowed to wear that number until their career ended.Why retire numbers when someone is still wearing it?
Occasionaly a team will retire a number after a player has left the team. Harold Baines had his number retired by the White Sox and he actually went back to the Sox for a short while after it was retired.





As far as 42 is concerned, only players wearing it on the date of its universal retirement were permitted to continue its use until they retire. Mariano Rivera of the Yankees is the only one left.