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!"