Why doesn't glue get stuck in the bottle?

There are two basic reasons:   [1] In order for glue to set and solidify, it must dry out. Latex and water-based glues harden by losing water, either by absorption into a porous substrate (the surface to be bonded) or by evaporation into the air. The glue bottle, at least if it is capped tightly, seals in moisture.
[2] Different glues are formulated to adhere to particular substrates. If the glue does not have a chemical adhesion to the substrate, it will not stick.

Even when the cap is left off, and the glue does lose water, the adhesion is still spotty. We can see this effect with the cap of many glue bottles. In most cases, dried glue can and does cake onto the tip after repeated uses. But we have to point out that the adhesion is "tenuous," and one can easily clean the top while still wet and remove the glue completely. http://www.straightdope.com/columns/read/2257/how-does-glue-work