Filmmaking is an expensive proposition. For these multi-million dollar films, you're looking at shooting schedules of 4-6 months (or more). For every one of those days, you have to cater enough food to feed your entire cast and crew. For a crew on a big budget production, you're looking at upwards of 250 people (or more). Not to mention the cast, both your stars (which do not come cheap--for A-List actors, many of whom are in these big budget films, you can be looking at 20 million dollars or more for their talents alone) and day-players (which, on big pictures, could still be someone famous enough to earn far beyond SAG-minimum). These films probably have multiple cameras (when filming action, you could be talking 15 of them) which cost thousands upon thousands to rent a day (remember to include the costs of lenses and especially film stock in these figures as well). You want to bring trucks or cars in for an action scene? Those have to be purchased or rented. And if they're going to be destroyed in the script you either have to have a number of exact copies available, or you better spend hours (or days) preparing (read: time spent paying actors to stand around) and be covering it from a lot of different angles (multiple cameras again).
And that's only production (which, often, baloons past schedules and budgets, which can't account for the random occurances of a film set). Pre-production budgets are often massive for these special effects-laden films as well. And it's no surprise if you have to hire massive art departments, etc. (or small ones--but then you have to give them a lot more time and end up spending money anyway). And, with dozens of writers on these assembly line projects (or just a couple of A-listers who earn millions), you're not saving any money there, either.
And then there's post-production. CGI ain't cheap, especially if you want it fast (and usually you do, in order to meet impending release dates that you've advertised endlessly--more financial burden--in order to hype up your film) and in the massive quantities intended blockbusters require.
And this doesn't even include the costs that studios don't advertise (huge advertising budgets--The Dark Knight probably had an advertising budget in the tens of millions, from the looks of it), striking thousands of prints (film prints cost thousands...multiply that by several thousand and you're not skimping), and yielding a good portion of your box office gross to theaters (notice they always report grosses--never profits).
But why do studios keep making these films? Because we keep going to them. We go to them in huge quantities. We go to them more than once. And we buy them on DVD. More often than not, they get us to buy them on DVD more than once. And that doesn't include the profits incurred from the toys, the merchandies, the fast food restaurant promotions, etc. that these films also bring with them. If you think the millions that the latest Batman movie has made in theatres is something, those numbers probably don't hold a candle to action figure sales.
Have budgets gotten a little out of control? Most definitely. The things Hollywood productions spend money on are often asinine. But, considering the massive profits that (more often than not) come at the end of the line, can you really blame studio executives (who fund films to make money...these people couldn't give a damn about cinema as an art--unless they can find a way to sell it) for not giving a damn about spending more than they should here and there considering the windfall that comes afterwards?