The 50% goes over all fields but the numbers in computer science are somewhat in that range. Computer Science has different issues than humanities and theoretical CS has not quite the same issues as the rest of CS. Certainly we lose several students to start-ups and high-paying jobs. But what about the ones that just have trouble in grad school.
Perhaps they lack the temperament to work on their own (which undergraduate work does not test as severely as graduate school does), or perhaps they lack, say, the mathematical chops necessary to succeed at advanced physics. But there will be a number—and if admissions committees do a good job, it will be very small—who won't be able to finish because they're not up to the demands of the task.Having read through many graduate applications through the year there are very few, perhaps on average one or two a year, that will clearly succeed through graduate school. Almost without exception those students go to MIT or Berkeley.
For the rest of us, you have a choice. You can either take someone who will probably work their way to a Ph.D. but with uninspired research, or those you can take a risk with a student who might have strong potential. Some of those students become great scientists, some of them flame out. You get a higher attrition rate by taking risks but that's not a bad thing.
If you do take a risk in admissions you need to encourage students to "pursue other opportunities" once you realize they won't make it. That's a process that too many of us try to avoid, so we don't take those risks as much as we should.