Mostly because yawning is involuntary and the face is so complex. There are 42 different muscles in the face, you use 11 to yawn, controlled by 6 of the 12 cranial nerves. Not all of these muscles are easy to engage on purpose, in the “correct” sequence. Kind of like how it’s hard to fake a smile convincingly without a lot of practice. Most people forget then eyes are involved in a genuine smile.
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