In the Pac 12, if every team plays 9 games, that's 54 games and 12 teams to be rearranged if the starting date gets moved from the last week of August to the 3rd week of September (and then still fixable if it gets moved again to the last week of October)

If you also have to arrange the non-conference games, now you've added 2-3 dozen games among 2-3 dozen opponents over whom you have no authority to say "hey, we need to move this game to this date". That team may not be able to do that (because they're playing another different opponent over whom you have no authority) and even if they can, they may have other reasons they don't want to do it.

If you cancel those games piecemeal as things come up (so you let USC play ND late in the season, but you cancel all the week 1 games) then you end up with a very unbalanced conference, including some possibly cancelled conference games.

I also agree that they may decide to cancel all of football and that this is just the first step. That will be unfortunate, but is certainly plausible. But the chance of playing a full college football season starting as normal was already nil before the Big 10 announcement. The chance of playing a 9-game conference season starting in September or October is probably 25%, but it's something. Might as well get the cancellations out of the way now so that the affected teams (BYU in particular) can figure out how they're going to adjust.