This is essentially my take. A large part of human morphology is driven by sexual selection.

If you (ie, your genetic code) are looking for a partner to reproduce with, health is likely the biggest factor to consider. You don’t just want someone who is healthy, but someone who has a long history of health. Having long hair allows your potential mates to check health history. So as other humans hair gets longer, if yours stays short, you are essentially saying “I don’t wanna talk about my health history”, which is a turn off.