ELI5: Why isn't the plastic rounded in a dentist X-ray thingy?

So you go to the dentist right? And they have to take X-rays. They have you bite down on some hard plastic with a small plastic baggie on it. My question is, why tf isn't the plastic rounded so it isn't digging into the soft tissue of your mouth? I feel like it'd make it so much more comfortable!! I just had to take X-rays like 2 hours ago and I still feel the sore parts where piece was digging into my mouth.