Home Assistant has a lot of moving parts, all the add-ons and extra user stuff. So they provide a docker image with everything you need, they also provide a full appliance install for easy setup.
If you did all that as a package install, you would complain about all the dependancies and if you didnt install the right version of something Home Assistant might not work at all
I used TVHeadend in OSMC and found it a pig to setup at first, but it works great once you have it all setup