What would you do? Everyone knows that some of the "top contenders" have more interviews than they know what to do with and they start to be more picky after they get an offer which can happen as early as Ocober 15th. So, schools have an incentive to woo the "top contenders" as early as possible in the process.
Also, as I mentioned recently, and for better or for worse, some schools will stagger the invitations so that the dates are chosen and locked in and not subject to cancellations and requests to move things around which can happen if the dates provided are 12 weeks in the future vs 3 weeks.
Also, let's say that we are going to read 4,000 applications over the cycle and half of those come in the first week. Of the 200 I'm going to read, will you dump 100 of them on me during week 1? It is likely to take me 8 weeks to work through that bolus and it makes more sense to send them in packets of 25-30 with two week deadlines than to make a huge indiscriminant dump. And if you are going to send smaller packets, you, the administrative boss, can prioritize the "top contenders" so that they are first on the block to get interview invites.
See how it works?