There are a lot of benefits to having only one kid.
It's cheaper. You can trade off on downtime with your partner. Eventually you find a routine that works for all of you, and the birthday parties and sports practices. As your child gets older, there's so much more you can do. So why on earth would anyone want to start over again with a baby? Add another to the mix?
The transition from one child to two can be daunting for parents. I remember Thatboy being very concerned he wouldn't love Thatbaby as much as Thatkid. How could he? His heart was already as full as it could get.
Personally, I was unconcerned. I don't think of the heart as something with walls. It's more like a balloon, put more in it, and it just continues to expand. For me, the transition to a family of four was almost...easy.
The adjustment from 2 people to 3 is a big one. Your world as you know it changes. Suddenly there are less hours in a day, and so much more to do. There is much less spontaneity. There's also an additional schedule to factor in. Despite what anyone tells you about sleep, there is no way you can understand sleep deprivation until you experience it for yourself.
The adjustment from 3 to 4 is not nearly as life altering. You've figured it out by then. And instead of changing your world for the new addition, you find a way to fit them into the world you've created. You've gotten used to a life with no spontaneity, and you've realized that no matter how bad sleep deprivation is, it will not last forever. You also have an additional set of hands to help out. Not with changing a diaper or anything that requires actual skill, but a small entertainer which gives you a minute of breathing room before the screaming of "MOOOOOMMMM MAKE HIM STOP" starts in.
Which is not to say that two children are easy. You will wonder how you ever complained about lack of time before, because now the hours go by even faster and the to-do list is longer. While you may not be called to entertain the children as often, you will be just as necessary to break up death defying stunts and acts of destruction to bodies, furniture, toys, or anything that doesn't move fast enough. Man on man defense becomes a real thing.
But I think the thing that makes it all doable is experience. Your first child is a scary, learning experience where you have no idea what to expect. When you survive that, you can survive just about anything. Even adding another child into the mix.