Major issue is Community service and tutoring/helping kids lack involvement. If you can be fully invested and involved then that's definitely better way to do. But most people can get much more involved easily when it's thier own children.
I would feel so much more limited and bounded in what I can do in the world, because my own financial future and my own mortality would be way more relevant with dependents.
That said, I didn't have my first kid until my late 30s and every thing I did prior to that seems like a pointless waste of time now. YMMV.
When you start talking about how they could have done literally all of those things by adopting any one of the bajillion kids that need it, most people will openly tell you that they would never consider it and don't feel even slightly hypocritical about it.