But the reason, to my understanding, why this is important is the cost to bring up a baby that's born with severe mental issues is a lot of money.

Human lives have a cost, it varies a bit on how you measure it, but around $1 million in western countries.

So if the cost to bring up this child is more, you are effectively killing someone.

So by stopping the creation of this costly life society is better off and lives are saved.

Certainly the concept a baby suddenly becomes a life just after being born is very flawed.

So Singer doesn't take the side you believe because he believes life don't exist at that point. He is not arguing for just killing someone.

If you scientifically prove a baby is a person he will change his opinion immediately.