For major companies that are profit-driven such as Apple or Microsoft, operating system development takes a major backseat as these companies have already carved out a sizable market share and don't need to invest much to maintain their dominance as they have self-sustaining app ecosystems for their OS which continue to draw users reliably.
Sure there are people innovating everyday in operating system development like with Haiku and Redox. Many Linux and UNIX distros are very active as well.
But for the major players, it's low priority, as it gives low return to shareholders. Which means the overwhelming majority of users won't benefit from most of these new innovations until they become sufficiently innovative that they become incredibly enticing (likely to expand the market share of a major player) or become either a market threat to a major player by being enticing to app developers leading to a them developing their own self-sustaining ecosystem of apps which draws users away from major players thus creating meaningful competition.
Which is fine, there's no reason you should be. But it means you're barking up the wrong tree.
But I would argue that for a company like Apple, their next iPhone release takes priority and will get a significantly larger allocation of their resources than the next OS level feature.
Overall, I think OS research is probably at a more interesting point in history than mobile hardware. There really is amazing competition happening between Linux/macOS/Windows and the users are getting those benefits. Research into secure computation and encryption is being integrated at the hardware level, security research is delving into lots of interesting problems w.r.t. badly behaving programs and also programs that behave correctly but attempt to perform malicious actions (i.e. buffer overflows vs keyloggers), and as lame as it sounds Linux's massive kernel improvements (I'm thinking specifically about their work on mutexes) have allowed them to become one of the quickest OS's in history.
It also brushes up against the question of what we're even calling OS research. The secure enclave was an iPhone first piece of hardware, but its concepts, development, and execution were certainly done by people who I would call OS researchers.
Case in point - though not a topic of this particular discussion, neither of the three were designed with security in mind. Most of the security features are, in my opinion, bolt-on afterthoughts. This is just my opinion though.