Because
1) most consumers have no idea what a NAS is, and even when they do, consumers have been conditioned that all their data is automagically beamed to the Apple/Google cloud at any and all time there's an internet connection without any user involvement beyond entering their ID when they unbox their phone for the first time, so it's impossible for a third party device or service to compete with this level of out of the box integration and convenience
2) NAS devices aren't made by Apple and Google so NAS integration into these ecosystems is a second class experience at best, and Apple and Google will never make NAS devices as they're incentivized to get you to pay for their cloud storage subscriptions for your data. Plus, as a a cherry on top, this way they can silently data-mine you as well.
Basically the industry is moving, or we can argue that it has moved already, towards subscriptions, where you never really own your music/movies/data but have access to it as long as you pay your monthly/yearly fee, because this is so much more lucrative for big-tech than getting you to buy commodity HW like a NAS and physically owning your data.