You make a number of good points. This isn't at all straight forward.
But parents having a baby together in any circumstances are not creating a child intentionally to do one of the most traumatic things possible to a child of immediately removing it from its mother for adoption.
If you adopt a child, you are intensively and invasively screened through a very long process which you may not pass, and your suitability for parenting is thoroughly assessed, and you must undergo training: not for general parenting skills but to understand the needs of a child who has experienced removal from birth parents. This is because, as is explained to adoptive parents from the start, adoption is rooted in loss. It is nobody's first choice; not the parents and certainly not the child's. The child is not a blank slate, but someone with a history and attachments and a life that has to be valued; the birth parents are not vanished from the child's story. Much trouble is gone to by adoptive parents to support children with this, including children removed at birth, to understand this, to be supported with feelings for the parent they did not know and were removed from. And adult adoptees speak of the difficulty of coming to terms with this removal, and how hard this is. Children born of donor insemination are expected to have rights to history, to contact, because of the damage done by removal of all history. Children are continued to be exposed to even highly abusive parents, sometimes past the point of safety, and while this is far from ok it is done based on the research that not knowing your history, not having a relationship with the person you grew from, can be damaging to the child and future adult, sometimes more so than protecting the child from contact.
And yet with surrogacy, the child's rights are removed at a stroke, and the parents have no such assessment, training, a requirement to demonstrate that they are able to put the child's needs ahead of their own and to meet this trauma. In this case, the child has been conceived with the outright plan of creating this trauma in the hope it just won't matter very much.
It's highly questionable. And that's before all the ethics around the use of women's bodies, human trafficking, whether it should be possible to buy or commission a person.
I take your point about anyone can birth a child regardless of how unsuitable: my point there was that stories about altruistic surrogacy may sound lovely, but aren't always, and the drive to do this is not always the lovely one it looks like. The nice stories and the nice outcomes should not mean it's unkind or wrong to look at the grim and very questionable aspects, or regard poor outcomes as just collateral damage to good intentions.
At one point shipping kids off to Australia was thought a lovely and altruistic, modern and progressive thing to do, all good intentions and wonderfulness. Now we look back on it and the lifelong damage it caused, and it's obvious that a whole lot more questions should have been asked, and a lot more caution involved.