Philosophers typically assume certain "fixed points" in arguments that they then refuse to budge from under any circumstances. It is a mental dysfunction, a kind of inbuilt rigidity of mind, that is peculiar to the breed.
For example, there is an actual argument in the philosophy of science that is based on the supposition that there is a substance that is "identical to water in every respect" but "is not H2O but rather XYZ".
Philosophers take this "argument" quite seriously, and get rather testy when you point out that the premise is a contradiction: if something has "all the properties of water", then that must include properties under electrolysis, and under electrolysis water decomposes to 2H and O, not X, Y and Z (whatever those might be). If you say this to a philosopher they will reply, "Yes, but what if it doesn't?" as if that was a coherent statement.
So the answer to your question is, "Because if we allowed ourselves to discard one of the premises we'd have a much harder time motivating this whole ridiculous discussion, and since our livelihood depends on such ridiculous discussions, we're not about to let that happen!"
Without disputing your main point about how philosophers typically behave, I want to clarify the is-water-H2O argument. As I understand it, it's making a point about how we use language: people have been using the word "water" to refer to various puddles, lakes, oceans, beverages, etc for a long time before electrolysis was discovered. We know now that all those instances have being-H2O in common, but it's not a necessary property for the word "water" to apply: if some other chemical fulfilled the everyday requirements for water (being clear, wet, thirst-quenching, etc) without being H2O, it would be "water," and if a particular variation of H2O doesn't fulfill those requirements (^2H2O, heavy water, is poisonous) then it's not water. See "Water is Not H2O" (Michael Weisberg, 2003). http://www.phil.upenn.edu/~weisberg/papers/waterfinal.pdf
I have a suspicion that many philosophical arguments, probably including the knowledge argument, are ultimately about language use - in this case, words and phrases like 'knowledge', 'physical' and 'physical knowledge'. Is this what Wittgenstein was going on about?
Yes, philosophers have such arguements: Water on some alternative universe without H2O structure vs water on our earth with H2O structure.
Even here, the issues are related to metaphysics, not so much about chemistry:) There is a jump from empirical world (the world we are in) to possible worlds. Here is the article: Is water necessarily identical to H2O, by David Barnett http://spot.colorado.edu/~barnetdb/my_papers/Barnett_Water.p...
> "Because if we allowed ourselves to discard one of the premises we'd have a much harder time motivating this whole ridiculous discussion, and since our livelihood depends on such ridiculous discussions, we're not about to let that happen!"
So saith the Amalgamated Union of Philosophers, Sages, Luminaries, and Other Professional Thinking Persons, eh?
For example, there is an actual argument in the philosophy of science that is based on the supposition that there is a substance that is "identical to water in every respect" but "is not H2O but rather XYZ".
Philosophers take this "argument" quite seriously, and get rather testy when you point out that the premise is a contradiction: if something has "all the properties of water", then that must include properties under electrolysis, and under electrolysis water decomposes to 2H and O, not X, Y and Z (whatever those might be). If you say this to a philosopher they will reply, "Yes, but what if it doesn't?" as if that was a coherent statement.
So the answer to your question is, "Because if we allowed ourselves to discard one of the premises we'd have a much harder time motivating this whole ridiculous discussion, and since our livelihood depends on such ridiculous discussions, we're not about to let that happen!"