Here's what I think... people, and that includes millennials, aren't stupid.
Going to a football game takes effort. You have get to it, which for some folks means a long drive, for others it means a shuttle bus, or a long walk.
Going to a football game costs money. Sometimes a lot.
Going to a football game often means sacrificing the better view you get on your 60" HD TV set.
While we're all nutty, diehards here, most people aren't like us. College football is a diversion, an outing, at it's best an event.
And here's the dirty little secret... most games aren't all that compelling. When you're the fan of a really good team, you romp over most of your schedule. When you're then fan of a really bad team, you get crushed repeatedly. When you're the fan of a mediocre team, who knows what you're going to get.
So if you're not a diehard, and the view is better in your living room, and there's no hassle, and it's cheaper... what is the burning incentive to go see your Seminoles pound the crap out of Florida A&M or Wake Forest? Now, when your Seminoles are playing Clemson the stadium fills up, because it's an event, and the experience is worth the hassle and cost.
Basically, when every game is on TV, and watching on TV is cheaper, easier and often better than watching in a stadium, and a good chunk of home games aren't particularly interesting to all but the diehards... why are we surprised less people attend?