My guess is that it stems from lax firewall defaults. "Allow port 53 - [tcp/udp/BOTH]?" (Yeah, I know that DNS can also work over 53/tcp, but it's rare compared to the 53/udp volume)
It's not as rare as it used to be a couple decades ago. If you block tcp/53 you will find a surprising number of things breaking as record sizes have increased over the years.