Because you have to maintain that local media server and go through the process of adding in the media you want to see and removing media you are done with if the disk is full.
As opposed to Netflix where you press a button and you're watching something.
People out there aren't much like the people who post here - when they get home from their crappy job they hate, finally make dinner because they can't afford food delivery, kick off their shoes on the old couch they bought at a yard sale and turn on their 15 year old TV they want it to just work. They don't have the interest or energy to fuck around with things that many here find fun and interesting and exciting.
With approximately 90% of the HT setups[1] I have seen, I can downscale a 4k video to 1080p and have the owner of the HT setup not be able to successfully ABX the difference.
1: I'm using the term "HT setup" rather broadly, as the primary location in a residence for watching movies as a group; it includes e.g. people who don't own a TV and watch movies on their laptop sitting on a coffee table. Setups where the display covers over 40% of the FOV (where 4k definitely makes a difference) are somewhere in the top 5%.
Netflix 4k is 15 Mbit.
So unless I see people mentioning the media, I am always weary of the comparison.