Why are there no dragons?

For Darwin Day, it seems appropriate to write a post about evolutiion.

Evolution seems to have produced a wide range of surprising abilities.  In many cases the same ability has come about separately in different species at different times.

Sight seems to have evolved in tens of separate ways.  Many creatures share the optical band that humans are able to sense (around the part of the spectrum where the atmosphere has the lowest absorption), but other creatures have developed to use infra-red and ultra-violet.  Some are very sensitive to the polarisation of light and others like us are almost completely unaware of it.

Sonar seems also to have evolved in about four separate ways.  Even in mammals like whales and bats there is nothing in common between their organs to suggest common evolution.

More surprisingly, electricity has been co-opted by completely unrelated species of fish, and Bombardier beetles have even evolved ways of creating explosions by secreting fluids which are unstable together.

But what of things that seem never to have evolved?

No animal has naturally evolved radar.  Perhaps the other options are good enough for short distances but much cheaper in terms of energy.

But fire is another property that is noticeably and more surprisingly absent from the animal kingdom.  This is the stuff of legend, with fire-breathing dragons being common in folk-lore.  Nobody serious suggests that these creatures really lived, even though there are creatures that create fuel (as cattle create methane in large quantities) and as we have already seen, electricity is generated at the voltages needed to make sparks to ignite the fuel.  The potential was there.  So why are there no dragons?