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.