If bananas are just potassium with extra ingredients thrown in why don't they catch on fire when you

For the same reason we can eat high-sodium foods without them bursting into flame on contact with water.

Potassium metals or sodium metals are different from potassium/sodium salts, and it's the latter form that is in bananas, if I recall my science correctly. The salts don't react violently with water in the same way that the metals do.

One of the fundamental themes of biology is that structure determines function. Even a simple change of a fold in the tertiary structure of a protein can cause massive differences.

Basically, if something’s built differently, no matter how small, it behaves and reacts differently.

Also, Potassium reacts violently with liquid water, not just oxygen or nitrogen.