I’m a software developer and have developed for Mac and iOS. That’s not how it works. Developer builds are out there for a reason and companies use them. It’s not a “huge waste of time” by any means – it SAVES time from being late with an update when the release comes out.
Sure, Apple (or any other company releasing a new OS) can change something between builds, but there’s also a specific roadmap communicated at WWDC, stuff known in advance (new libs added, documented changes to existing APIs and services etc.) that has to be adopted early. It’s not like Steinberg has to re-architect large parts of Cubase to keep up with any new beta. And even devoting a couple of programmers would make a big difference in their update coming sooner.
Other, more diligent companies, already have updates out, for equally or even more complex programs (including DAWs).