If your circular needles have flexible cords and are long enough, you don't need to buy DPNs. Magic Loop, and Traveling Loop are easier to learn than fiddling with multiple DPNs.
I've made dozens of hats on circular needles, right down to the tiniest preemie hats. I don't even own DPNs.
Its what we get used to I think, I have tried magic and traveling loop, even 2 circulars. Always get in a muddle but with DPN's I am fine.