Why does High Sierra remove my Login Items?
I have lost all my login items too after upgrade to macOS 10.13.1.

You could take a look the plist file at ~/Library/Preferences/com.apple.loginitems.plist, it may still contain names of login items, then add them back manually. I recovered mine in this way.

I guess 10.13 has moved login items config to somewhere else but failed to migrate old ones.