Many of the main characters in the saga of Dragon ball, the work of Akira Toriyama, are not humans but beings belonging to the alien race of Saiyajins. Saiyans are far stronger than humans and have become the race of the strongest fighters.
During the various series Dragon ball, Goku and Vegeta never change hairstyles unless Vegeta wore shorter hair in the non-Canon series Dragon Ball GT.
It's not strange for the characters to keep the same look in their franchise, especially when it comes to anime and manga. However, in Dragon Ball we saw the main characters change from saga to saga several times. For example, Krillin at the end of Dragon Ball Z has hair, even if it returns to his signature hairstyle at the end Dragon Ball Super. Yamcha changes her cut several times in the saga, and Bulma changes style often too.
It is Vegeta who tells us about this typical Saiyan trait after training with Trunks in the Spirit and Time Room for a year. While Trunks hair has grown very long, Vegeta's hair hasn't changed an iota. Bulma will ask for an explanation, and Vegeta will explain Pure-blood Saiyans retain the same haircut they were born with all their lives. However, this theory doesn't work with a beard. In fact, after completing the training, we saw both Goku and Vegeta with beards.
Who knows if Dragon Ball Super's new bow we'll see our protagonists show off a new hairstyle! In the meantime, we look forward to new Dragon Ball Super-themed announcements at Jump Festa 2021, mark your calendar on December 19th and 20th.
