Cause the verb is inflame and means to place a flame in on on something, as opposed to not flame. The problem being in both inflammable and the issue the OP of the image is the notion of re-bracketing. Rather than think of it as [inflame]able it gets converted into in[flammable] in the minds of new readers/listeners which flips the in- prefix meaning to not rather than in/on like it is supposed to be.