why are we so really getting new hairs in ff14?

[Ameela]

as the title say why we rarely getting new hairs?

We're not, they just shifted their design philosophy with them so rather than getting one or two hairstyles added to the defaults for free in every patch you now get hairstyles as a reward for content instead.
Not a bad idea at all, cosmetics is one of the most alluring types of rewards in the game and it goes double for hairstyles which are in much lower quantity than all other forms of character customization in the game.