Why do cells not store glucose?
The issue here can be explained by colligative properties:

In short, colligative properties are properties that depend upon the concentration of the molecules, but not upon the type of those molecules. And osmotic pressure is a colligative property.

That being said, have in mind that 10 molecules of a huge polysaccharide in a given volume or 10 molecules of a small monosaccharide in the same given volume will have the same osmotic pressure.

So, let's imagine that the cell needs a given amount of glucose. Let's say 1000 molecules, just for an oversimplified example. The osmotic pressure can be calculated by (assuming Van't Hoff factor is 1):


  &pi; = MRT


Where M is the molar concentration, R is the ideal gas constant and T is the absolute temperature. 

Thus, the osmotic pressure created by those 1000 molecules can be calculated taking into account their molar concentration.

However, If we simply polymerize those 1000 glucose molecules into a huge polysaccharide (be it starch, glycogen or cellulose, it doesn't matter)...


  1000 molecules of glucose &rarr; 1 molecule of starch  


... the molar concentration will be 1000 times smaller, and the osmotic pressure will drop 1000 times.   

As the actual numbers are way higher than this simplified example, we can clearly understand that cells avoid storing glucose to prevent a deadly osmotic pressure, in which the cell bursts due to the amount of water that moves in (specially animal cells, which have no walls that could prevent the membrane from rupturing).