That would be simple to do so, but life is not simple: what happens in Egypt affects Israel and the entire Arab nations. We are all one world, and there are wide consequences of the outcome of what happens in Egypt. In times of civil unrest, intervention and mediation by stable other governments, such as the US can generally provide, is necessary to prevent world war, a disaster no one in any country would want. In the intervening 2 years since you asked this question, Egypt is STILL not settled on a form of stable government and is in highly active civil war, something the Egyptian people I'm sure can all agree is the worst-case scenario. People need to be safe sending their kids to school, going to the grocery store, and going to work. Egypt is somewhat unique in that its economy is primarily based on tourism (as of 2008, US and Russian tourists were the primary tourists). This has all but halted since the civil war began, hence Egypt has little to no income and needs assistance from other countries. As a Super Power in the world, if not THE Super Power, the US attempts to provide aid to other countries who request it or to otherwise take action to maintain global peace, with albeit varying results that can be much debated by most people around the world. Time moves on, and history unfolds, and only time and history will tell what will happen in Egypt.
One specific reason the US and the West in general are concerned is the central role that Egypt plays due to its position on the globe. The fate of Egypt highly affects the fate of the Suez canal, through which oil flows throughout the world, and also it affects Israel's safety and sovereignty. While the US could easily be oil-independent from the rest of the world, the same cannot be said of most other countries with fewer natural resources and differing technological capabilities. In other words, if the Suez canal closed, the US would have to supply not only itself but much of the rest of the world with oil/energy/technology to produce energy. If Israel fell, there would rightly be cause for concern that a world war would break out.
Regrettably, this is a highly complex issue with many, many factors in addition to those I've stated. Just as the world would intervene if it looked like the US was about to fall, the US tries to intervene to keep other countries from falling. Egypt, however, has a complex history that led to its current state, which appears inevitable upon 2 years' retrospect.