The mother doesn't just 'carry' a child.
A mother feeds her baby, nurtures it, develops and grows the child in a deeply interconnected relationship; her body carefully calibrating and adjusting to cater for the needs of the foetus/baby as it grows. The baby learns her voice, smell, heartbeat in the womb.
'babies can tell the difference between the voices of their mothers and the voices of strangers ... listening to mom may increase a preterm infant’s autonomic stability, and promote healthy weight gain ...
https://parentingscience.com/how-do-children-respond-to-a-mothers-voice/
She then labours, and births the baby. Various other interdependent processes play out, including rooting for the breast, latching, grooming, and many other instinctive and unlearned acts and actions performed by both mother and baby, processes which build on each other to create as a whole the safety, responsiveness and care that a child needs for a good psychological start in life.
All of this is the most intricate and (I'd say) amazing process or series of processes and interactions, all of which speak to the importance of the mother/baby dyad, which is the foundational relationship of our lives. As mammals this is just how we have evolved - a strong mother/baby bond is very important for the wellbeing of a baby.
To imagine that a woman is some kind of anonymous oven that can carry a baby and then pop it out and pass it over with no effect on the mother or the baby is a very sad and grim misunderstanding.
Our understanding of adoption means the trauma of mother/child separation is understood and acknowledged. In surrogacy, this trauma appears to be being strenuously denied. Babies born of surrogates do not seem to have this understanding or acknowledgement of their trauma - I suspect because most of the parents buying babies don't want to admit that they've knowingly and deliberately inflicted this trauma on the baby they're charged with caring for.