Being pregnant and giving birth changes us as human beings. Our perception of self can, and usually is, altered irrevocably. These moments in our lives are some of the most crucial because all future generations are affected by these moments.
If the birthing person views their experience negatively, they can turn those feelings inward. This can cause their self-esteem to suffer. The negative self-talk, doubting their abilities, can then make it difficult to trust their capability to parent. Parenting is hard enough as it is; throwing in feelings of personal mistrust makes it infinitely harder. Parents struggle to make choices or trust their instincts.
Sometimes when a person suffers trauma, they can disconnect from things that remind them of that trauma. This can/does happen with people who have traumatic births. They may have a hard time bonding with their baby because their baby triggers them into a fight-or-flight response. This emotional removal from their offspring is felt by the child and that child's feelings of self-worth then come into question. Why doesn't my parent love me?
Put these two things together: a parent who doubts their abilities and a child who doesn't feel loved. This vicious cycle is hard to break. No one is at fault -- the problem is the cycle. This is why birth matters. Our feelings of self-worth and our ability to parent depend on feeling empowered, heard, respected, and loved.