Because just like with every other living organism on earth, our goal as a species is to ensure that we continue to exist.
However, (((society))) have convinced a lot of people that choosing not to have children is acceptable. And while this may be true to a very small group of people (for various reasons), for the majority of people, it's not acceptable. You should aim to have at the very least 1-3 babies.
In NS Germany, there was significant encouragement for people to have children and there was actual support. However, nowadays, this burden can be overwhelming so a lot of people just choose to not have any children.
I believe that having children is extremely important, and if you are a financially secure white person with a white partner, I'd even say you should have as many children as you possibly can, especially considering what Abyssos mentioned. That is a great point, in Europe there are people coming from all over Africa with one goal in mind which is to mix with whites.
We are literally on the brink of extinction and people are still wondering whether or not they should have babies.