It's pretty easy to refill glue sticks if you buy the right glue. Either as pre-made inserts or buying the glue in bulk, melting it over hot water, and pouring it into empty glue sticks.

It's very uncommon though since glue sticks are so cheap and it's just easier to throw them out when they're empty and buy new ones. Obviously not a good situation sustainability wise but we live in a throwaway society so it's par for the course. As such it also tends to be more expensive than just buying a new glue stick unless you're doing it on a sufficiently large scale.