well... i guess it is like asking why are some doctors obese and why do some doctors smoke? things that make you feel good in the short term aren't necessarily what is good for you in the long run...
also, i hope some of the "tan" doctors are using spray on tan or the lotions you can buy in the store rather than tanning in the sun or under tanning salons.
i'm currently sporting a tan even though i don't want one be cause i'm too lazy to put on sunscreen when i swim in the mornings. bad bad bad.