Why is the atmospheric pressure so high on Venus?

Because Venus has such a dense atmosphere which is some 100 times thicker than the earth's. Evidently, Venus is sufficiently close to the Sun that the little carbon dioxide it had in its early, Earth-like atmosphere caused the surface to warm up and leach out more carbon dioxide into the atmosphere. This in turn caused the atmosphere to get warmer in a so-called runaway Greenhouse Effect. Very quickly, much of the carbon dioxide that was originally trapped in the surface rocks wound up in a thick atmosphere which now regulates the surface temperature at about 882 F.