Don't really see the benefit of waiting until Friday and doing it all at the same time.
I am guessing they want to be able to send out the offers and then disappear for half term rather than face a barrage of calls!
I suppose also there might be some perceived advantage to a school in offering earlier - the child/parents might get attached to the one that’s “in the bag” - so it’s fairer if they all go at once. Plus they all coordinate acceptance dates (eminently sensible and thank god they do), so same offer date ensures same consideration period for all.