Because the greatest costs of declining fertility aren't visible disasters, but missed opportunities. The millions and billions of people who are never born won't share their ideas with the world. They won't help spread the fixed costs of idea creation, product variety, and infrastructure. And they won't enjoy the gift of life. (If you're already objecting by listing the upsides of non-existence, think again). Low fertility isn't bad because we'll lose what is. Low fertility is bad because of we won't gain what could have been.
This is from Professor Bryan Caplan at the Library of Economics and Liberty (Econolog) (ht Professor Art Carden)