Why don't all knife-weapons have 2 sharp sides?
Better structure! Fundamentally speaking, the sharp sides are thinner and therefore weaker, and thus more likely to shatter. A second, very slightly more nuanced answer is that only one side actually is used to attack. Third reason is that single edged blades are easier to make and easier to maintain (only need to sharpen half the edges! Half the work!)
By knife I assume you mean swordlike- (plus knives/daggers and swords have the same fundamentals with a few nuances that I won't yap about now, so if you DO mean knives this'll still answer your question.) so here's some sword examples. (plus some random facts.)
Yapping in order of images, left to right. Straightswords, oddity of the german executioner's sword, and curved swords.
Straightswords- (E.g- European and Chinese broadswords)
Two sharp edges means an extra edge should the other wear out
better for stabbing- this is why they tend to be straight, since straight swords excel at straight-motioned thrusts
sharp edges are thinner than if they were flat/dull. This means they're weaker, with less support. They're more likely to chip
dual-edged blades can slash and thrust effectively.
Because it's sharp on two ends, it cuts into something in a thrusting motion better. There's no dull hindrance so it's simply an easier cut.
unrelated fun fact! Straight swords are more likely to shatter in general. Hitting a too-solid object can shatter the blade since the impact is firmer, while a curved blade is more likely to glance off before the impact causes more damage.
Straight swords have more uniform force throughout, while curved blades have varying levels of force depending on where the impact is on the blade.
German executioner's sword- (E.g- german broadsword. Duh)
Solely used for slashing- ideally a clean, single slash to the neck to lop off a head.
Thrusting is not what this sword is used for, ever. Therefore, no need for a sharp tip. The tip resists shattering since it's thicker.
Despite being ceremonial, these blades are seldom extensively decorated. They tend to at most have carved holes or words.
Curved swords- (E.g- indian talwar and japanese katana)
flat edge is thicker and therefore stronger
fighting forms generally don't utilize the flat edge
one can steady or guide the blade by placing hand on the flat edge- not as feasable with a sharp edge
some 'single sharp-edged' blades feature a small sharpened length on the unsharpened side. (look at the talwar) This aids with the initial cut during a thrust, allowing the rest of the blade to travel through the body more smoothly than without it
blades without the small sharpened length feature thrusts at a more directed angle, so that the blunt edge is behind the sharp tip, allowing for a smoother initial cut
unrelated fun fact! curved blades align with the natural arc of a swinging arm, especially with cavalry.
only one side needs to be sharpened. The chances of using the inner edge is extremely unlikely. Why waste time and effort sharpening it? Having a sharpened inner edge is usually a waste of effort and potential durability
Some blades have sharpened edges ONLY on the 'inner' curve. Look at some greek kopises.
