As someone who puts this as a "want" and certainly not a "need" - the answer is really just management simplicity. That and HAProxy is generally a joy to work with.

I already have a nice anycasted cluster of HAProxy servers that already have the battle tested BGP session teardown logic/etc. integrated - I really don't want to have to use a different load balancing method to HA my UDP services and re-do large portions of that tooling.

It's certainly an annoyance, and being able to define traffic in the same configuration file/syntax/etc. makes it much more maintainable for an ops vs. engineering group.