I think you are jumping to the wrong conclusions. In fact certain low cost airlines encourage pax to check in for both their outward and return sectors at the same time, which is why they charge excessive penalties to those who don't. My question is with the legacy carriers who tend to insist on a maximum of 24 hrs or so.
sapperkenno: you are way behind the times and technology has moved forward. I can tell you that in the days when pax had no other option than to check in at the airport there were still many instances of them not showing up at the gate.
Edit: written before I saw ExXB's post.