Why Boeing 737 main landing gear wells have no doors? My question is about the Boeing 737 main landing gears:

When the landing gears are up you can see a landing gear door to open in most planes and then it closes. But for Boeing 737 main landing gear are not behind well doors at any time of the flight. 

Why are main landing gear wells opened during in flight?
To keep costs down. The extra friction from no gear doors is more than made up from the weight savings.