Lack of time or interest (or both) in: managing local DNS, using .home.arpa
and running own CA.
It's tricky (especially running your own CA in a proper way), and not everyone wants to do it. Also, running it in a proper way it requires knowledge, and some people don't have it...
Actually, distribution of your root CA certificate is not that difficult.
NOTE: this addresses strictly OP's question about LAN-only access. External access or varying devices used to access is a completely different story.
For one instance of app, it's possible to install it onto a single machine.
Things get tricky when you want to access the data from multiple devices. Even trickier, when several people want to access it. After a certain point, it's easier to have a "cloud" solution. And since "cloud" is just somebody's else computer, why not make this a computer YOU own?