Because I can run `ping` in a busybox container that knows how to do all the same service discovery as a more complicated fully fledged microservice. No extra libraries. No smattering of supporting services (on top of DNS, which you need anyway, of course). Setup and debugging is 10x simpler than any of the more complicated service discovery solutions.

And it just works. Simply and intuitively. With everything. Because it's been an IETF standard since 1983.