Well, when it comes to the bookkeepers being able to trumpet what the sales figures are, or speed, or whatever they like to measure these days, there tend to be established notions of what time period the figures come from, and I'd suspect they'd like it if digital sales matched. Of course, such dates/time periods tend to be somewhat flexible ... like how films will sometimes release on Thur. or even Wed so they can lump that in with "weekend" figures. Anyway .... generally speaking, it's probably usually some reason like that. Sometimes a company may be able to push for/market a same day release but probably it's not worth the effort most of the time.
Probably, there's an awful lot of careful massaging of figures which tends to take priority over everything else!
I think what's irritating is when there's a UK studio with a US publisher, such as Lionhead and Microsoft, in which case the UK still has to wait even for locally-developed games... The worst aspect of the delay in general terms is the tendency of spoilers to start leaking out in those first few days when everyone else is waiting to start playing.