For example: you're expected to follow the law even without knowing what the law says. When you want to find out what the law says, it's not easy--it's certainly not available in a standardized format. When you want to interpret what you find, assuming you find it, that's not easy either. Courts interpret things in new ways all the time.
The federal court system charges you to access public information contained in court proceedings, with limited exceptions--that is, if you even know where to look for it. See http://www.thinkcomputer.org/20120209.pacer.pdf. The interface is terrible and hard to use. The way in which you write lawsuits is obscure, counterintuitive, and creates additional needless work.
In addition to all of these factors, and perhaps because of them, lawyers (especially at big firms) have institutionalized fraud. It's taken for granted that legal billing is often fraudulent. If you charge $500 per hour and your system only resolves to the tenth of an hour, that means if you spent four minutes writing an e-mail, you can charge for 0.1 hours, or $50. But really you only did $33.33 of work. That's a nice cushion. But what actually happens is that an attorney might do 45 minutes of work and round it up to an hour--even though that work is formatting in Microsoft Word that the client could have done; or printing out a Word document in order to scan it in as a PDF. Still seem worth $500 per hour?
For those lawyers not at large firms, they're covering expenses (such as law school) that are enormous. High rates are a necessity, and who would charge far lower than market rates anyway? It might be interpreted as a signal that something is wrong.
Of course, don't for a minute think that paying $800 per hour will get you a better lawyer than paying $300 per hour. It might. Either way, you'll be paying someone in a staggering number of cases to unscientifically guesstimate What The Government Might Do, when the answer is, "who knows?". That doesn't mean all lawyers are the same; some are definitely better than others. But it has nothing to do with price.
More lawyers could afford to charge reasonable market rates, and not work for large firms, if it weren't for the ABA mandating that you have to attend a law school (that results in huge piles of debt) or clerk for years (four in California) in order to join the bar. See http://www.nytimes.com/2011/10/25/opinion/are-law-schools-an....
Lawyers know, too, that you can't get rid of them (also thanks to the ABA), and so you're locked in. There's a monopoly on business representation, for example. See http://www.plainsite.org/issues/index.html?id=137. It's absurd.