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.