In this case, “the prefix ‘in-’ is not the Latin negative prefix ‘in-’, which is related to the English ‘un-’ and appears in such words as indecent and inglorious. The ‘in-’ in inflammable is an intensive prefix that is derived from the Latin preposition ‘in’. This prefix also appears in the word enflame.”
from www.bartleby.com
So don’t blame it on English, blame it on Latin! ;-)
Harp, I expected better from you!