Upvote:8
Because people use online check-in before they get to the airport, whereas with offline check-in you are by definition at the airport. They want to know if you are actually there at the airport.
They don't want you to be doing online check-in 40 minutes before your flight because you probably won't get to the airport in time. Even if you manage it they won't know you will be there which makes their planning harder.