In a word – politics.
A new report from U.S. PIRG (PDF, 1.8M) describes the political forces that cause politicians to favor ribbon cuttings over basic maintenance programs:
One thing is for sure: the deterioration of our roads and bridges is no accident. Rather, it is the direct result of countless policy decisions that put other considerations ahead of the pressing need to preserve our investment in the highway system. Political forces often undermine a strong commitment to maintenance. Members of Congress, state legislators
and local politicians thrive on ribbon-cuttings. Powerful special interests push for
new and bigger highways. Meanwhile, federal and state policies–which should provide strong guidance in the wise use of taxpayer dollars–often fail to achieve the proper balance between building new infrastructure and taking care of what we already have built.