Lillie There are all sorts of reasons for not treating children fairly and the OP gives one of them. Justice and fairness are factors more important than equality.
In my own family my grandfather left everything he had, which was, essentially just the house, to the unmarried daughter who lived with him and his wife and had provided the help and support they needed in their old age. He did this with the knowledge and agreement of his ten other children, all married and with homes of their own.
However I do think, in most cases, every child should be left something. I read a very poignant article by someone who had been very successful compared with the rest of their family and she had known about and agreed with her parents decision to leave their estate to her siblings, but found herself deeply upset, when this happened. She said she realised that to a certain extent, a will is a final act of love from parents to children, so no child should be excluded. She didn't want an equal share with her siblings, but she wanted a mention in the will and a bequest along side her siblings, even if it was only a token amount of money.