This is rather the point.
If the universe we experience is absolutely indistinguishable from a material, external one, and we can never experience, say, bobbing about in saline solution, there is no way to coherently say that we are "really" brains in jars. There is no way in which the universe behaves as though we are brains in jars; the ability to say "well, we might be anyway" is more a quirk of language than an actual, semantically valid statement.
EDIT: Note the use of the phrase "can never" in the above. If it is possible for there to be a break in the illusion, then the universe is not indistinguishable from a material one. In this case, we move into the realm of there needing to be evidence in order to accept the assertion of possibility, and the idea can be dismissed once we look around and see that there isn't any, rather than simply because it is worthless by definition.
Because some people are very, very silly.
You have this precisely backwards.
Materialism is not an assertion of the "true nature" of reality, as such questions are incoherent and undefined. Materialism is the conclusion that the universe and all things in it are made of a single substrate, which we refer to as "matter".