Because the school has to order them and proctor them for you. The school has to be certified to give the tests which is why some schools only have AP or IB.
Fun fact though. Your school doesn’t have to offer the AP class to take the AP test. I just straight up asked if I could take the AP CompSci tests despite having no computer programming classes. I sat in a room alone with my counselor for a day while he administered the tests, boom done, easy 5 since I had been programming since before HS. I tried to tell some of my friends who were also aiming at CS in college but they didn’t ask.
No, but you still need to have a means to get there on test day. My school wouldn't proctor tests for classes I hadn't taken, and I only found one other school within an hour willing to do that for a student out of their boundaries. I jumped at the opportunity, but I could have just as easily not had it all if, for example, my family hadn't moved while I was in middle school.
Fun fact though. Your school doesn’t have to offer the AP class to take the AP test. I just straight up asked if I could take the AP CompSci tests despite having no computer programming classes. I sat in a room alone with my counselor for a day while he administered the tests, boom done, easy 5 since I had been programming since before HS. I tried to tell some of my friends who were also aiming at CS in college but they didn’t ask.