Most mammals seem fine with shorter hair (usually denser though). The only other case that comes to mind of very long hair restricted to a specific body area is that of horses manes, which to be fair I’m also not totally sure what purpose it serves. Many equines seem fine with shorter manes and tails that don’t have such long hair, like zebras. They swish flies away all the same.
Some animals have long hair but it covers most of their bodies (like llamas or yaks maybe), it’s not restricted to a small area. Other animals have denser or longer hair in some areas, like lions, but this serves a purpose (protection of the neck and head) and even then the length ratio between these longer hairs and the rest of the fur isn’t as skewed as that of humans.
So, why? I get that hair on the head helps protect it from the elements and sun, but why so long? Some humans can grow hair longer than their own body length, which is remarkable, and without doing any fact check I’d say we are probably among the top 5 species with longest hairs ever. Is it just a showoff feature like a peacock’s tail feathers, an indicator of overall health? Or does it serve another function as well?
I didn’t mean to type this wall of text…, thanks for coming to my TED talk
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.