There’s a couple of reasons.

The first is that the population is substantially larger than it was during the Ottoman period. At the time of the French incursion in 1798, the population of Egypt was estimated to be between 4-7 million people; in the 1917 census, it was just over 13 million. Today, it’s around 100 million. Add to this that only 6% of the land can be used for agriculture, and you can see the issue: there’s just no way that a country that was already considered to be a “bread basket” can increase its output 20 times over.

The desert cannot be used for agriculture — the geography isn’t the same as, say, next door where Zionist settlers in mandate Palestine were able to “make the desert bloom.” In Egypt, the surface of the eastern and western deserts are situated around ten meters above the water table, which means it’s not simply a matter of “bring water and watch it bloom.” The only places that can support agriculture are depressions in the bedrock where the surface is close enough to the ground water to allow things to grow — hence, the word used in Egypt for “valley” and “oasis” (wahhat) are the same!

Under Mubarak, there was a plan to try to link depressions in the Western desert via canal and create a Second Nile Valley, but the cost is enormous (and appears to now have gone to the new capital), and never really took off.

The second reason is that, under British guidance (from 1876, and then occupation from 1882), the Egyptians were encouraged to grow cotton as a cash crop. The country was massively in debt to British and French banks, and the interest was in making as much money as possible — cotton was king (also, since most cotton mills were in Britain, they could guarantee its purchase). Wheat and rice were grown in lesser quantities - and, since they were of good quality, these, too, were sold for export while cheaper products were imported, usually from India, for domestic consumption. (This caused a ruckus during World War I when import wasn’t possible, but the country continued exporting wheat).

Aaron Jakes’s new book Egypt’s Occupation (Stanford U Press, 2020) explains the economics applied here.

The current Egyptian government continues this trend: sell the better stuff for a higher price, import cheaper stuff, make profit.