If you want to understand why cutting the deficit is so hard, you can’t do much better than to look at the Business Roundtable. The roundtable is one of the more moderate big-business lobbying groups. Its president is John Engler, the former Michigan governor, and its incoming chairman is James McNerney, the chief executive of Boeing. When roundtable officials talk about the deficit, they use sober, common-sense language that can make them sound more reasonable than either political party. But the roundtable is actually part of the problem.