This is exactly the mindset that leads to waste, inefficiency and bloat. If you can save even 5% on bandwidth, which is a limited resource, WHY NOT DO IT? And this is way more than 5%.
Because h.265 requires hardware decoding to be even moderately watchable and the vast majority of devices that people currently own are not capable of decoding the latest h.265 videos. Netflix could force people to upgrade which would cause them to buy new decoding hardware and the waste you mention would shift from wasted bits on a network connection to wasted hardware in landfills. Unless, of course, there is a standardized way of querying hardware to determine which codecs and maximum resolutions/bitrates are supported by hardware decoders but I'm not aware of such a feature.