It's an extra setup and device for a customer to manage and if they want to view the data on their other devices away from home (a large part of the marketing of Ring et al) you get into difficulties getting the data from the 'home NAS' solution, it's 10x easier to just have the devices stream to the cloud 24/7 and deal with accessing it from there.
While I think you're exaggerating the difficulty of doing this without involving an external service, I fully acknowledge that it's easier to have someone else take care of the whole thing.
The most difficult part is making sure the connection to fetch recordings when requested by a person using the app is reliable. That needs the server or your phone to be able to contact the NAS on the other side of your ISP and your router. That's hard and a headache to trouble shoot compared to just having the camera stream to some central server.
