I know of eight research studies that have shown that stepmothers are generally considered less successful than stepfathers in developing positive relationships with their stepchildren. The most difficult relationships are between stepmother/stepdaughter ie, they are characterized by less affection, less respect, less acceptance and higher levels of conflict and stress than other stepparent-child relationships. One study reported that only about 20% of adult stepchildren felt close to their stepmothers, compared to about 50% for stepfathers. In that study, two-thirds did not even consider the stepmother as a parent at all.
Several studies have addressed the stepmother's role from an economic perspective. While the presence of a stepfather helps to raise the family's standard of living, the presence of a stepmother has a negative impact on the resources devoted to the stepchild. Three recent studies by the same research group concluded: "for complex reasons stepmothers invest fewer resources in children than biological mothers do" Children raised in families with stepmothers are likely to have less health care, less education and less money spent on their food than children raised by biological mothers. (Bear in mind that the economic status of remarried families was no different overall from that of intact families).
We often think that the greater number of benevolent adults present in a child's life, the better for the child. However, research strongly suggest that this belief does not apply to stepmothers' involvement; that often they are better off without a stepmother.
Of course, the data refer to group differences. Of course, there are some excellent stepmothers. Of course, some stepmothers and stepdaughters get on tremendously well.
But generally the OP's observation reflects the research data.
(BTW, I am a stepmother.)