I'm still learning too, but the short answer is that the seller of the options needs to manage their risk if the option is exercised. If you sell a call, you are promising to provide a stock on demand at a particular price. The most straightforward way to ensure that you are able to sell a stock in the future is to be holding the stock now. If you don't already have enough of the stock in your portfolio, you need to buy it.

A longer but excellent answer is here: https://youtu.be/9eDqmOPSs9Y?t=908