Round robin is hard when you're doing things concurrently and distributed, because state is hard.

A server is taking requests 30 at a time in separate threads. To add a sequential step for picking which target is next, you need to add a synchronous piece of code that all of the threads depend on- a bottleneck. That's doable, but it adds complexity.

And then you have (let's say) 30 servers making calls. Do they each use round robin load balancing separately, or do they try to coordinate? That's difficult.

"Pick two randomly" requires no state. Every thread and server can run the same algorithm without regard for each other.