The problem is that what is often perceived as blatant diving isn't. If any athlete is running fast, they are working right at the limits of their balance, and the slightest outside factor can upset that balance. This is why, in, say, athletics a "tumble" can seem much more dramatic than it need be. In football, the act of trying to avoid a tackle can be enough to make a "balance" player fall over, as can such daft things as finding an unexpectedly soft piece of ground. When balance is lost it is very difficult to regain, and the natural instinct is to try to fall without injury - hence some players may look as if they are "diving" when in fact they have lost balance and trying to land safely. Then there are the other sort, who appear to make a complete meal of things - Carroll the other week, for instance. If you are not a natural balance player, loss of balance can just turn you into a windmill of arms and legs; this looks obviously playing to the gallery, but isn't.
The only cases you can 100% say are diving are those where the "diver" has been standing still and flings himself over (I seem to remember a few from the recent ACoN being on Youtube).
The corollary of this is that it is often impossible to say, even with technology, whether a foul has actually been committed. Just as a slight change in ground pressure can have a catastrophic effect, so can the slightest touch - and that would be a foul.
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