The word 'maa' is the first word spoken by a newborn baby. For that baby, the one who shows extra care and nurture him every day, the one who embraces the child in her arms, laugh with him, respond to every word spoken, preparing food for him is the only one GOD to him.
Moreover, parents are the one who is physically present with the child every time without even asking to do so just like God is watching all of us what we are doing, or what we want without us asking from him to do so.
God is only one for everybody as we are so much dependent on him for everything that is happening, we worship god a lot, believe in the spiritual path because we humans believe that if no one stays god will stay with us always. So just like god, our parents are the one who stays with us whatever the situation is and will always appreciate us for whatever we are doing and stay by our side only.
Another reason for this can be that in India we people believe that we are a creation of God but we cannot see him so the ones who gave birth to us that are our parents are our god. They are also considered our first teacher. And a teacher is also our god.
The life of the parents revolves around the lives of their children it doesn't matter they are right or wrong. When we become grown-ups we'll see our parents made so many sacrifices for us just to make us smile, make our life comfortable and an adjustable background.
At that time we all understand why our parents are given the name of god. So we should respect what all we have as life has given us so much to do for them. Nothing is worth watching our parents feel proud because of us.