Three strong reasons:
1) As Schopenhauer (and Buddha) said, the average human experience on this earth can be characterized as "suffering."
In other words, net negative value. The vast majority of people have unfulfilled lives, then drop dead. It's thus unconscionable to bring another human to this world just to suffer most of the time.
2) Most people are ill-fit to be parents. My parents didn't know how to parent, and neither did yours probably. Most people are winging it, creating mentally imbalanced monsters (see Jamin).
3) This Earth could use a little lower usage of resources and lower asset prices.
To make this work, we need to enforce vasectomy on men around age 14, which can be "undone" if and only if the person passes some qualitative and quantitative tests (e.g. psychological health, net worth, income, etc)