We don't. There's no central authority that makes message brokers; it's a group or individual who wants to make a new one that has some specific characteristic

We don't. There's no central authority that makes message brokers; it's a group or individual who wants to make a new one that has some specific characteristic that isn't replicated elsewhere (anything from throughput and latency through language and creators). Similar question: Why do we need so many JavaScript frameworks. Answer is the same: we don't.