Why is my oven bulb so hard to turn?

The socket is probably corroded or has debris in it, using oven cleaner sprays is very hard on that socket because it is not coated with porcelain enamel like the rest of the oven. It is bare metal, usually aluminum. Cleaning it can be tricky but if you endeavor to do it, unplug the oven first or turn off the circuit breaker if the oven is hardwired and wipe it with a damp (not wet) sponge or cloth and let it dry thoroughly before plugging the oven back in. Then you can spray a spritz of cooking spray on a paper towel and wipe it onto the bulb threads (not the button on the end though) before screwing it in. If it were not an oven you could use a little rub of bar soap on the bulb threads, but that will impart a flavor to your food in this case.

Carbon can form in a hot electrical environment. Use a PLASTIC vacuum nozzle to suck out the socket. OR use a clean dry toothbrush to clean it out by rotating the toothbrush, and then vacuum out the socket. Wear gloves for protection against shock and broken bulb glass.

If either the socket or bulb base is aluminum , it has probably galled. No nice fix; turn off power and twist, you will likely need to replace both socket and bulb.