because there is no disc to "authenticate your ownership" to the Xbox, so it will attempt to reach some remote server to look at your "account".
You can play it offline. But only on one console. Xbox has the concept of "home console" or "primary console". You can download a game to it and as long as the console checks in every so often (once a year or so) the license will be valid on it.
All other consoles you log into must be online to validate the license.
So it's not truly offline play but you could play the campaign offline as it doesn't need to do a license check on your primary console. Note that this only works to keep that console offline - you put it online and it'll refresh the licenses you have so if it got transferred it'll stop working. Or after a year, which ever happens first.