why are glycogen and starch most suitable than glucose as storage products?

Glucose is the more usable form for the plant and hence it cannot be stored in the form in which it is readily and easily usable, therefore it is stored in the form of starch (not readily usable) which is a polysaccharide. There are other advantages like large polysaccharides take less space, it does not affect the concentration of water inside cells as it is insoluble.