Beyond that, I'm sure that senators leave hearings all the time, often thoughtlessly, but often for completely valid reasons -- like other hearings or other meetings that have been previously scheduled. This isn't like Hollywood, where if you're an star people coordinate their lives around you and sit through every word you say. United States Senators sometimes have other things to do.
But beyond even that, it helps to know your audience: Sen. Mark Kirk (R-IL) had a stroke a few years ago, and only somewhat recently returned to his duties in the Senate. If anyone on that committee was someone who supports such a life-debilitating disease, it was likely Mark Kirk. Rogen should have been aware of that, and known that this is someone to keep close to you, not push away. For all we know, Kirk left because he doesn't have the stamina yet to sit through all hearings, but felt this important enough to show up. I have no idea if that's the case, but the point is that Rogen didn't know if it was so, either.
Again, I think Seth Rogen's speech was terrific. And I like that he was calling out senators publicly to get them to pay attention. I just think that he didn't handle calling out Mark Kirk as well as he probably should have, and if it took a step back in retrospect, he might agree.