etymology of the word 'inflammable'?

Inflame means to cause to burn.  Flame means burn.  Well, a thing that can be caused to burn is effectively as likely to burn as a thing which is burnable.

The noun forms come into existence via different verb forms.  The action part, the verb itself, does have a different meaning.  The noun part loses that distinction, because the distinction is only in the act, not the result.

While the "in" prefix most often implies the negative, there are times when it implies or increases the meaning of some words.

Because it just does.