This is far more normal than putting goop into a tyre. Like, it's been that way since the tyre was invented.

I believe it has to do with the size of gas molecules in the air, and the smaller molecules find a way of permeating through the rubber inner tube. I think people who use CO2 to inflate their tubes, have greater issues with deflation over a shorter period than those who use normal air.

Car tyres don't have the same issue to the same extent, mainly because... well, have you seen how thick a car tyre is?