Because there are always so many different chemicals in each different pill, and if you used one chemical to substract the mdma from one pill that same substance could reuin a totally different pill.
Not normally. Tablets usually contain binder/s (e.g. MCC, corn starch, etc) which are fairly insoluble in water. This is done to avoid the tablet absorbing water and becoming crumbly/ mushy. Some pills however, do contain other inactives which are soluble, in which case they would be harder to remove using just water.
Amphetamine salts including MDMA are water soluble. If there is only one active present that is soluble in water, and the binders aren't, then an extraction should be pretty straight forward. It's when meth, MDMA, ketamine, caffeine etc, are present in combination that it becomes tricky.
If say a pill did contain 2 active ingredients, both of which were soluble in water, then other solvents or combinations of solvents may be used. Various techniques can be employed which make use of individual solubility properties. E.g. compound x is slightly soluble in water, compound y is very soluble. As a higher temperature usually increases solubility, sometimes cooling the water first can result in only compound y readily dissolving, while the other less soluble compound does not, or only slightly dissolves.
As mentioned by mike0, there are many threads that discuss this. Do a forum search, perhaps also using terms; extraction, recrystallization, solvent etc. and if you can, find a copy of the "Merck Index" or the "CRC handbook of Chemistry & Physics" (both publications contain solubility data). These are available in e-book version, the former also as a torrent.