"Why can't it be two people, assuming that they are both responsive?"
People smell/taste/feel different, a lot of research was done via the old orphange set up and the deprortees, where various care givers saw to the baby's needs.
hardly anyone challenges it.
Father's groups constatnly challenge child law (where this fundamental bond is protected).
Academics then have to be carrying out on going research, which builds on what is already written, to show good reason why birth mothers and fathers are not given identical rights, in practicality (but in essence they are).
The studies are culturally crossed and different types feed into each other, so adopted orphans from other countries, in different aged sibling groups etc.
Some of the studies are around how we as adults form attachments to our baby's and talke that into account. It is a lot more in biological basement then an "insecurity".
I don't know how aware you are OP of the brain development that doesn't happen if attachment isn't formed and neglect takes place. The frontal lobe doesn't form etc. So a baby's needs not being met can cause brain defects, we do not yet know everything about healthy psychological development.
We know that a lot happens straight from birth and the 6 months theory is way off mark.
A lot of what we know is by getting it very wrong and not thinking it was important how many foster carers a baby had, as long as needs where met in other ways.
It has never been disputed that one Primary Carer is vital.