That’s because the setting is “preserve Live Photo settings in Camera”, i.e. if you turn off that setting, Live Photo will come back automatically after some time since it won’t preserve your previous Live Photo settings (i.e. off) in Camera. To prevent Live Photo to come back automatically, you have to turn on “Live Photo” switch in the setting for Camera.
P.S. although Live Photo is a feature that Apple love a lot which make it very hard to turn off, but it is still not as hard as turning off taskbar preview in Windows.