Why does glue not dry while in its container?

Drying is one of the main reasons for this, yes. But there can be others as well. There are some types of glue that do "dry" with time, even though they are sealed. They don't dry so much as harden, and will no longer flow. Glues based on epoxys are one example. The reason is that such glues not only depend on evaporation to 'dry', but a chemical reaction that results in the glue hardening. The same can happen with cement for example. Take a small amount of cement powder out of the bag, add water, then seal it in a container. After a few days, open the container and you'll see that the cement is hard. Not because anything evaporated, but because the chemical reaction that produces cement proceeds, after you add water, whether anything can evaporate or not.

It's the air! When it's out in the open and air hits it it dries. When something is sealed in a container there's no air and the liquid doesn't dry out.

I think once the air hits it than it drys up. But being in a container with a lid on it not much air is able to circulate through it thus keeping it from drying out. Once it is spread out it has more drying capability because it is thinner and any water now has more ability to evaporate making it dry out.