In addition, a few other factors:
a) Stealth Rock, which makes up a huge part of defence in this game. It is very hard to take hits when your HP is wiped by 25% (or more) every time you switch in.
b) Fire doesn't resist many good offensive types. Steel and Bug are bad offensive types, and Grass isn't far behind. Resisting them isn't noteworthy, and done better by Steel.
c) Fire does resist itself as well as Ice, which are good offensive types. However, Water also resists those types, and has more merit as a defensive typing beyond this. In other words, even where Fire is good it is still outclassed.