The manner in which this trope appears in fiction is difficult or impossible to replicate in reality for a number of reasons. Whenever you cut through something with a blade, such as a fruit or vegetable, you are severing the connections between two parts of the object, which makes it possible for them to go in different directions if acted upon by an outside force. Meanwhile, the edge of the blade has percussive force at the moment of impact and continues to push against the medium until the whole cross section is cut through, which has the effect of transmitting velocity and vibration to the object as it's being cut in half. At the same time, and even for brief moment after the edge exits, the flats of the blade are transferring a certain amount of force to the object through friction with the inside of the cut. The two halves of the object will acquire random velocity and spin from countless little variables acting upon them. The only way for the object to stay together would be some kind of strong suction, adhesiveness, magnetism, etc. between the two parts that pulls them back together at the same rate that the blade cuts through, and not many things that you can cut through with a knife or sword have such properties. Certainly not humans.
Furthermore, in order for a blade to cut through an object it needs to be stiff enough to hold its shape. A blade for fighting has the additional requirement that the blade be tough and meaty enough that it won't get completely wrecked by blade-on-blade contact. As a result the blade cannot be microscopically thin, and one might even intentionally use a wedge-shaped cross section to split the object apart as it cuts, i.e. the opposite of what you'd try to achieve for a Diagonal Cut. While all swords do tend to push a wound open as they cut through, it's funny to note that this is especially true of the classic katana with a shinogi zukuri blade cross section, which ironically is the favorite sword for performing a Diagonal Cut in fiction. Now, the way to get around this problem of the blade's thickness wedging the wound open would be to use a blade so thin as to be flexible. The urumi is an example of this kind of Whip Sword. However, whipping or flicking the blade at an opponent does not provide the consistent pressure needed to cut through limbs, and such a weapon would instead work by just cutting the flesh to ribbons, with all that flopping and twisting about making a bloody mess instead of a clean, straight cut. One can reintroduce stiffness to an ultra-thin blade by means of a frame which pulls on both ends to keep it in tension. The same solution is used in hand saws designed to cut a very narrow kerf. This is of course very impractical for fighting, since the delicate blade would still be very vulnerable to damage, could not be used for thrusting, and would be limited in how deep it could cut by the size of the frame's "throat." The extreme version of the whippy blade method is Razor Floss, where in theory you could minimize the friction from the flats of the blade by just not having flats, and as long as you pull on both ends strongly enough the target will be cut. Unfortunately, it's only really practical as a clothesline trap, or to garrote somebody you already had the drop on. To wield Razor Floss as a weapon in a straight up fight you'd have to figure out how to grip or handle it without cutting up your hands—let alone control whiplike threads that could be just as dangerous to you as to your opponent—and you'd need to wrap up or lasso your opponent in order to cut them up, which is rather more complicated than hitting them with a sword. Besides, the cuts inflicted by wire actually tend to be rather messy. None of these methods really provide the desired results, and the differences are, if you'll excuse the pun, splitting hairs.
