Simultaneous theatrical and home release have been tried a few times, most notably with "Pirates of Penzance". You won't see too much of it, because the theater owners have raised a big stink. Justifiably fearing that this might cut into their business and hey need time to recover their costs. They have told the studios that if a studio announces home availability for the same day or too near the theatrical release date, in most cases they simply won't book it. The studios also want to build in some time so that they can get the maximum movie gross. Since Skyfall was the 2nd highest grossing picture worldwide, you can see their interest. On the other hand, for movies that aren't super-blockbusters, the studios want to make the delay fairly limited so that short-attention-span Americans will still be interested in the flick.
Since there's little indication that waiting a limited period of time hurts DVD sales (would you buy or not buy Skyfall because it came out 90 days after release as opposed to 30?), the studios and theater owners have come to an accommodation. The theaters are "protected" for a certain amount of time after theatrical release before the home release. Sometimes, a movie is so popular that it runs so long that it's still in theaters by the DVD release date, but that's rare.