It called Ficke's Law of diffusion. Basically, you have a high pressure system inside the tube and a low pressure system outside the tube. The tube isn't perfectly impermeable to air inside the tube and it works it's way out of the tube through the rubber. This happens with all high pressure systems but metal containers tend to be less permeable and retain air much, much longer.

The rubber of bicycle tubes is particularly permeable with respect to carbon dioxide. Fill a tire with CO2 and you'll be pumping it up the next day. That's why CO2 should only be used for emergency filling (lazy filling) of tires on the road. Fill them with real air before the next ride 'cause CO2 is expensive!