You can still have other friends. It just takes more effort to build and maintain those relationships.
Once out of college it is the people inside your routine that are your friends, be it a work or a hobby. Upon having a child your routine changes yet again and will change rapidly as the child grows. It is the difficulty of maintaining relationships through the shifting routine of life that is so difficult.
You need one aspect of your routine that has stability through the change. Mine is physical activity.
Not saying it isn't hard, and not saying it isn't a challenge at times (I certainly don't see my friends in person as much!) but the idea that having kids forces you to be friendless is absurd.
In the past, lets say baby boom years, almost everyone had kids so the family friction coefficient was very similar between different friends.
But these days it's quite different. We typically don't leave our kids at home alone any more (GenX represent!). There is also a much larger group of people without kinds (DINKs and SINKs) that have far more free time than those with kids. So for them the equation changes.
They have the cost of inviting you and the friction you experience. Or the cost of inviting the childless friends they know. Or the cost of the myriad of entertainment options we have these days.