There are more metrics than just pixel density that they need to optimize for. HDR, WCG, refresh rates and latency, aging/color stability, ... 4k is good enough

There are more metrics than just pixel density that they need to optimize for. HDR, WCG, refresh rates and latency, aging/color stability, ... 4k is good enough for most needs, so the demand is elsewhere. Plus the gaming market is limited by what GPUs can render.