There are two major factors:
Population and
geography.
1.
Population: There never had been tens of millions of people living along the Nile River and the Nile flood plains at any one point of history. Meanwhile, the population count of Egypt today stands at
110 million people. In fact, the population of Egypt has been steadily rising non-stop since the 1950s and 1960s.
2.
Geography: There is this map -
View attachment 105063
Notice that pretty much the entire population of Egypt lives right along the Nile River, and concentrated on the Nile Delta. The rest of the country is pretty much inhospitable arid deserts. You could even say that the entire country of Egypt depends on Nile to survive.
Considering the two factors listed above, the Egyptian population have already went way past the maximum resource carrying capacity that the land of Egypt could ever realistically sustain. That's why they have no choice but to source much of their food supply overseas.
Therefore, for Egypt to realistically resolve this issue, there are only three ways:
1. Enact population control measures; and/or
2. Reclaim the lands from the arid Sahara desert for the farming of crops and livestock, which could take decades to complete; and/or
3. Desalination of the water from the Red Sea and Mediterranean on a mass scale, and channel the water to alternative farming methods such as hydroponic farming, which would be very expensive.