Why do you think most of Egypt was mainly desert?

Most of Egypt is desert because it lies in the rain shadow of the Ethiopian Highlands. The prevailing winds in the region are from the north and northwest, and as they rise over the Ethiopian Highlands, they cool and drop their moisture as rain on the Ethiopian side of the mountains. By the time the winds reach Egypt, they are dry and hot, and they don't produce much rainfall.

In addition, Egypt is located in the subtropics, which means that it receives a lot of solar radiation. This solar radiation heats the air and the ground, and it also causes the water in the soil to evaporate. The high temperatures and low rainfall make it difficult for plants to grow in Egypt, and this contributes to the desert conditions.

However, it is important to note that not all of Egypt is desert. The Nile River Valley is a lush, green area that is home to a large population of people. The Nile River provides water for irrigation, and it also helps to moderate the climate.