Why do the brothers take their parents for granted while their parents were alive?

This is an interesting question. I think that most young people do take their parents for granted in some ways. I would argue that in this novel, each of the boys loved their parents. Their reflections are simply day-to-day interactions that families have without knowing what day will be their last chance to talk to a loved one. In Jacqueline Woodson's novel Miracle's Boys, each of the three brothers spends time reflecting on their last encounters with their parents.
Ty'ree, the oldest of three Bailey brothers, is the only one who was present when their father died. In chapter ten, he divulges to Lafayette the last words he said to his father:

"'After Daddy got out of those wet clothes and climbed into bed, I came into the room. I asked him if he thought the dog would be okay.' Ty'ree smiled. It was the saddest smile I'd ever seen in my life."

Ty'ree goes on to say that he wanted a dog so desperately that the dog was his main concern. As he has gotten older, he realizes that his dad was more precious to him than any dog. But it was a normal reaction for the developmental stage Ty'ree was in when his father died. Ty'ree now feels guilty about it because he wishes he had said something more meaningful. How could he have known it was the last time he'd talk to his father?
Lafayette deals with similar guilt. He was the last one to interact with his mother. The morning he found her, he was asking her for his breakfast. He is also dealing with the regret of wishing he'd said something more meaningful. But again, he couldn't have known she was going to die.
Charlie deals with a different type of guilt. He wasn't present the day either of his parents died. He was a young child when his dad died and was taking a nap. He was in Rahway Juvenile Detention Center when his mom died. So I don't think they take their parents for granted in any way beyond what is normal for children. They weren't outwardly disrespectful to their parents, and they didn't exclude them from their lives. They are simply wishing for more time with their parents and dealing with the grieving process.