Sell outs are due in large part to the great majority of the tickets being held by long time season ticket holders. Most of those season ticket buyers know full well that the 'market' price of the tickets is considerably lower than the 'face price' the ticket office sells them for. The excess of the face over the market is effectively a donation. If tickets weren't packaged as season bundles, there would a number of games every year that would be practically impossible to sell while the few highly watched games would bring a considerable premium.
This has always been the case and attendance is impacted by much more than the general fan interest or disinterest in watching the team. Even in 1995, at the peak of NU power and the pinnacle, arguably, of what should have been fan interest, tickets for some games were still not easy to resell even at half of face price. Weather, kick off times, competing public events, activities such as fall harvest,or local events, etc. impact the ability of individual fans to actually attend games. Total tickets sold will always (there can be exceptions and were back in the late 60s and early 70s when sneaking into the games was quite common) exceed attendance. Over time, as people have grown larger, the comfort and convenience of attending has declined. Traffic, parking,, and so on also have made attendance even more expensive. TV coverage also impact attendance, while tickets are still being sold, there are more and more going unused or given away. Many also only go to the first half and then go 'tailgate'. Even on bright sunny Sept days when the team's season outlook remains mroe hopeful.