It’s important to remember that ‘Ancient Egypt’ existed for a reeeeeally long time so many of its weapons evolved before or around the development of quality metalworking. In the earliest periods, bronze working was not developed to a point where Egyptians could produce long, sharpened blades like a sword requires. Axes and other weapons developed from stone Neolithic tools were much more practical. The khepesh, or sickle sword, is hypothesised to have developed from one of these, the epsilon axe (Terence Wise, Ancient Armies of the Middle East). There is evidence to suggest that it was introduced to Egypt by one of the Canaanite groups they had contact with. Throughout the history of great empires, this type of exchange where massive empires import ideas from smaller groups living on their fringe as their own technology stagnates is not uncommon (Chris Harman, A People’s History of the World). The khepesh, despite the curve, is also NOT harder to produce than a straight sword. The khepesh, like the axe it evolved from, needs to be strengthened and sharpened along a much smaller surface. The khepesh is also fairly versatile, being employed for a range of tactics on the battlefield depending on the size of the weapon itself, several different sizes being found in Tutankhamun’s tomb. Straight stabbing was usually achieved separately, by a spear which infantry also carried.
It is also (probably) incorrect to suggest that Egyptian’s used curved swords exclusively. Daggers, rapiers and even larger swords were employed by Egyptian armies, particularly by chariot crews. While there is archaeological evidence to suggest their use in cutting through swathes of infantry by fast moving units, there are little to no Egyptian depictions of their use during battle which is rather curious (John Coleman Darnell, Colleen Manassa, Tutankhamun's Armies: Battle and Conquest During Ancient Egypt's Late Eighteenth Dynasty).
Agricultural cutting tools have always been the most common edged weapons. In forest societies this generally meant axes, and in agricultural societies, it meant harvesting knives. These were not glamourous weapons, due to their association with commoners, and it is rare to find ones that got the artisan treatment that helps to ensure survival into modern museum collections. For example, falchions (derived from L. falx for "sickle") were hugely popular weapons in the middle ages, but knightly swords tended to be better at surviving to the modern day. (Note that this remains true today - the machete or "bush knife" is certainly the most common sword-like tool still in widespread use, and it still sees regular use as a weapon, but almost nobody treats their machete like an heirloom weapon that is deserving of special care.)
The primary advantage of these weapons is that they are cheap and plentiful. They were made both straight or curved, and typically both straight AND curved in different parts of the same blade.
The curved parts cut better because the edge impacts the target at an angle, increasing the effective sharpness of the edge. See this diagram: cut A hits direct and does not benefit from blade curvature. Cuts B (swinging cut) and C (pushing cut) impact at an angle to the spine of the blade, increasing the effective sharpness of the edge. You can get the same effect from a straight blade by drawing it across the thing you are cutting - that's why drawing a kitchen knife across a tomato cuts it better than pressing it straight down. Curved blades give you some of that drawing effect on a regular swing, without having to "saw" it back and forth.
Note that forging a curved blade is actually easier than forging a straight one. Starting from a squared piece of metal, if you hammer one end/side flatter and flatter to get it down to an edge, that metal will spread out. The thicker "back" of the piece will not spread. The resulting piece of worked metal will have a thin, broad, curving edge, backed by a stout squared spine. This is why axe heads tend to flare out into a curving, rounded edge. Sword blades will do something similar if they are single-edged, which agricultural weapons tend to be. Double-edged blades require a lot more work, and are more difficult to use, since back-handed cuts and straight thrusts require more skill. That makes straight swords more of a specialty item.
Refs: I think I first came across the analysis of why curved weapons cut better in Hutton, perhaps The Sword and the Centuries although I'm away from my copy at the moment.
Related question: wasn't a very similar design also used by bronze age Canaanites? Was it a technology they imported from Egypt? How popular was the sickle sword in the bronze age near East, generally?
