It's simple. Because their triggers pivot instead of moving in a straight line. The US Army recognized this issue on their 1905 Colt contract pistols, and a few were retrofitted with the grip safety for testing and evaluation. It became an integral part of the redesigned 1907...reappeared in the 1909s and on the 1910 model...which was the 1911 prototype...and the rest is history. The 1911's grip safety is a drop safety...just as necessary today as it was in 1907. If it wasn't, it would have been eliminated long ago.