There are many orphans or children who are living in orphanages because their parents couldn't afford to keep them. People have a moral duty to care for the already existing children in need of a loving caring family rather than making new babies in an already overcrowded world.
If there is a moral responsibility to adopt existing children before using surrogacy, it would therefore also be immoral to have children naturally while there are already children in the world living without homes. Additionally, there are many barriers to adoption that make it difficult and it can be psychologically hard on both the birth and adoptive parents, as well as the child itself. To adopt is a huge undertaking.
[P1] It is immoral to go through the process of surrogacy while there are children already in the world without homes.
Rejecting the premises
[Rejecting P1] There is no reason having children through surrogacy while there are children without homes is any worse than having children naturally.