Something doesn't have to be an evolutionary advantage to persist in a population. If long hair doesn't impact fitness then it won't get selected against.

Long hair could also just be a sign of fitness in the same way colorful feathers are for some birds.

You’re misreading the process of evolution.

Not everything that is retained has an evolutionary advantage. It might simply come down to preferential choices resulting in the spread of the gene (say, horses’ mane - a female horse may prefer the look of a longer mane, resulting in that gene spreading and becoming ubiquitous, and a species-defining trait).

I think you are looking at this from the wrong perspective, I think you are discounting sexual selection as a basis of why humans have long hair. I suspect the advantage is less this helps me to survive, and more this helps me to find a mate.

Another response pointed out the advantages of having dense curly hair and how and why we evolved with that, I think our ancestors used that starting point as a means of signaling health, youth, and vitality. And from there different hair types evolved as an expression of sexual selection.

We evolved to have heads of long thick hair because our ancestors found it attractive. And to this day it continues to be used (subconsciously or consciously) as a means of determining the sexual health of a potential mate.

If you think about it, when we get old or sick the hair is one of the first things to falter, not having a head of long thick hair doesn't necessarily mean you are unhealthy, but it is also difficult to maintain a head of full shiny head of hair without having a level of health and vitality.

Tldr: I think to this day hair is used as an indicator of sexual health from a potential mate, having long, thick hair can signal that health from a distance, and would be a distinct advantage in spreading those genes.

There are two different things coming into play here.

First of all, hair length evolved before long hair did.

Modern humans originally evolved tightly curled hair. Basically like we still see in many African populations. It's thought that this was an adaptation to protect against the heat of the sun. Basically like an insulated sun hat. The longer this curly hair grew, the more protection our natural hat provided.

As homo-sapiens populations moved further North, this protection was no longer needed - in fact there was the opposite problem, it was cold and rainy.

Greasy straight hair offers an advantage over curly hair in this kind of environment. It acts like a waterproof blanket, preventing the skin beneath getting wet, and it dries more quickly. Heat is lost through wet skin significantly faster than dry skin, and in situations where energy sources might be hard to come by in winter months, this can be a disadvantage.

We already had hair length sorted, so it was simply a matter of reacquiring the straight hair shape.

Europeans got a leg-up in this regard - Neanderthals appear to have had straight hair, and interbreeding definitely occurred. At this point it's worth remembering that by the time anatomically modern humans evolved in Africa, earlier hominids were already living all over Africa, Europe and Asia. Neanderthals were very similar to modern humans and shared an extremely close common ancestor.

And this is how, and why, some homo sapiens populations have long straight hair.

Humans evolved with dense curly hair, which protects you from the heat https://www.psu.edu/news/research/story/life-air-conditioning-curly-hair-kept-early-humans-cool

I read/heard something recently that they found evidence that it has to do with bipedalism and keeping the sun off of you. Don’t need fur all over to do that if you stand up straight and grow hair from the top. Still, adaptation and trait selection is incredibly complicated and you probably can’t boil this one down into a nice answer.

Demonstrates health history and general safety over the last potentially many years, which is good for status and attracting mates. The capacity to shear hair once stood as a statement of technological capacity and combat effectiveness: "I have a sharp knife."

By the by, here is a picture of a really cute

Gutsick Gibbon has a pretty good video on this topic!:
https://youtu.be/EgZAVIJHTiM
The short answer is that it protects our heads from the Sun's radiation.

There is the fun aquatic ape hypothesis. It's a fun one, along with the stoned ape hypothesis. But unfortunately there's not a lot of actual evidence for either of them.

Funny idea what if its a remnant from the early tool use era where hair was used to make all sorts of stuff and people that had more long hair could do more stuff which increased their survival chance? Pretty sure thats nonsense but it would be funny imo

protection against uv radiation, keeping it dry?

I'm not sure it's the primary reason long hair evolved in humans and I'm not a scientist, but I do know that you can use it kind of like handlebars/reins when you're hitting it from behind. Only with their full, notarized consent, obviously.

mate selection, almost entirely. it's sensitive to body condition to the extent that if a woman sees the same hairdresser consistently enough they can somewhat reliably tell if she's gotten pregnant. it's also an easy indicator of social class and stability ie: do they have the time and resources to take care of it?

We do lose most of our heat from the head, so the original purpose might have been insulation

Doggy style

TLDR I, specifically, look more fabulous