Why are the Turtles never shown unmasked on screen?

[the fixer]

I've always wondered why the Ninja Turtles are never shown without their masks in the movies or TV shows while they appear unmasked quite a lot in the original comics...
There's no real reason why they would wear masks while they're chillin' in their lair, is there?

I sure hope we'll be able to see them unmasked a couple of times in the upcoming movie...