MQTT doesn't have the same semantics. https://docs.nats.io/nats-concepts/core-nats/reqreply request reply is really useful if you need low latency, but reasonably efficient queuing. (making sure to mark your workers as busy when processing otherwise you get latency spikes. )

You can do request/reply with MQTT too, you just have to implement more bits yourself, whilst NATS has a nice API that abstracts that away for you.