I suspect that birth parents just can't imagine how anyone could possibly love their child as much as they do.
I don't care if someone could love my child as much as I do, I still don't think that should trump their and my right to our relationship.
TBH I don't find it at all weird that my parents or MIL might live my kids as much as I do. Differently, obviously, but I don't see what that has to do with anything.
Regardless of how much anybody else might love, or fancy they love, my children, I am their mother and our relationship to each other (and to their father) is the primary one.
I chose to have them, I chose to bring them into the world and look after them and be responsible for them.
Even if somebody else could prove "stronger" love, I am still their mother.
I suppose a comparison is when I was in a particularly loved-up stage with DS about 6 months in and I used to pity birth parents because they hadn't experienced this amazing bond with a child not biologically related to them, that they hadn't experienced the amazing feeling of learning to love a child with every fibre of your being without biology or hormones to assist you.
You're right, that is a feeling that biological parents will never have.
I'm not sure you quite had "no biology or hormones" to assist you, but it's a fair point - a parental bond not based on the drive to pass on your genes. That sounds amazing.
It's easy to forget that with the adopted people in your own family, because an adopted cousin is really no different from a birth cousin. A friend's child is just a friend's child, you love them the same regardless.