The trinity of God Brahma, Vishnu, Mahesh represents 3 steps in our life: Generation, Operation ( Sustenance) and Destruction making GOD.
It is in the eternal 3 phase cycle for any existence. Bramha represents Generation or birth, Vishnu the sustenance and Mahesh or Mahadev the end or death.
If we see human life or any life, we observe that generation takes a little time, death even less than that and major part of our life comes under sustenance or survival and everyone strives to live more.
This way, birth is the least important activity for an individual. In fact, we are not involved in our birth. Almost the same is true for death also. The sustenance gets maximum focus. So, Vishnu is most celebrated God, found everywhere in every form ( Avtaar). Elongation of life is possible if we can avoid our end or death, so because of the fear factor, Mahadev or penalising power also gets some respect, particularly when we are in a panic . And his temples are also found everywhere.
Who does bother about the reasons behind our birth? That is a one-time phenomenon and none is interested in returning to that state again thus generation becomes least respectful activity and even if that happens, it happens in the closed-door environment, be it inside the womb , germination inside soil or manufacturing inside a factory.
Within our body, genitalia, responsible for procreation is to be kept always covered up. Nature, otherwise also keeps genitals in a hidden situation in the body . That is essentially Bramha’s organ in our body, important, yet invisible, and not worshipped.
Compared to that, Face in anybody, is responsible for sustenance (eating, breathing, seeing, hearing, smelling) and represents Vishnu. And Face gets all recognition and focus in life. It is used for identity, respect and makes up , cosmetics, ornaments are used to decorate it.
The head top part under our hairs also gets very less exposure. Most of the cultures insist to cover it up. That part, containing Brain, is our death centre, the Mahadev and it also gets some importance.
So, that is part of the evolution cycle!
Everybody thanks the driver of a car for a safe journey but no one even thinks to thank the workers who manufactured the car, nor you ever see them in your life!
So Bramha remains invisible and needs not to be worshipped openly!
PS: People can raise an objection to my views by saying that we do respect our parents for giving birth to us! But that is not true, parents are respected more for upbringing ( Vishnu’s role ) rather than giving birth( Bramha’s role). Everybody hates parents who leave the child on the road after birth.