There are several reasons to make less glue than they are able and all are in the interest of the company and not the consumers. It's an issue of supply and demand. If they
Manufacture a huge amount of glue, not only do they have to store it somewhere and outlay on the raw materials and manufacturing costs but they also run the risk of flooding the market and devaluing their product.
If they are able to say that they only have a limited stock they invite potential consumers to enter a bidding war or to pay whatever inflated prices the glue maker chooses to charge.
If there were any competition this would not be possible but having a monopoly on the product allows the manufacturer to control the supply according to demand and to turn a higher profit.
