wonder why glue doesn't stick to the tube
Glue is actually a mixture of an adhesive and solvent. When we put glue on a piece of paper, the solvent is exposed to air, and easily get evaporated. Remaining is the adhesive that sticks to whatever it's on. This is how glue works. the glue is sealed in the tube, the solvent can't evaporate, as it is not in contact with the air. So the glue stays in solution, and not especially sticky. That is why the glue inside the tube doesn't stick to it. Leave a container of glue open for a day or so, and we can see it stick to the walls of the tube impressively well.