Actually, they probably aren’t forgetting. Lol! It’s kind of humorous, but in this case evangelical Christians are probably correct that Jesus did not have long flowing hair. That he is depicted as such stems from a medieval conflation of Nazirite with Nazarene.
Jesus was a Nazarene, a resident of Nazareth, not a Nazirite, a member of a special Jewish ritual order in which men vowed never to drink alcohol, cut their hair, or touch corpses.
Somehow in the artistic world a few hundred years ago, people got the idea the two were the same, and so came to paint Jesus with very long hair.
We don’t really know how ordinary men in Palestine cut their hair. Like ordinary people everywhere, their lives didn’t get documented very well. We know elite men in both the Roman and Greek worlds that dominated Palestine cut their hair quite short. All we have to do to know that is look at statues and surviving frescoes.
Since Palestine had been dominated for centuries by first the Greeks and then the Romans, we presume the ruling class emulated their style of dress and grooming. Meaning they probably had pretty short hair. But we don’t even know that for sure.
Actually we don’t know much of anything regarding Jesus’s hair length.
Which makes setting biblical standards all the more bizarre. ;-)