Why is there so much iron?

The solar abundance of iron is a little bit more than a thousandth by mass. If we assume that all the baryonic mass in the disc of the Galaxy (a few $10^{10}$ solar masses) is polluted in the same way, then more than 10 million solar masses of iron must have been produced and distributed by stars.

A type Ia supernova results in something like 0.5-1 solar masses of iron (via decaying Ni 56), thus requiring about 20-50 million type Ia supernovae to explain all the Galactic Fe.

Given the age of the Galaxy of 10 billion years, this requires a type Ia supernova rate of one every 200-500 years.

The rate of type Ia supernovae in our Galaxy is not observationally measured, though there have likely been several in the last 1000 years. The rate above seems entirely plausible and was probably higher in the past.

Iron comes from exploding white dwarfs and exploding massive stars(Wikipedia).

(One of many amazing images by Cmglee )
Periodic table showing the cosmogenic origin of each element. Elements from carbon up to sulfur may be made in small stars by the alpha process. Elements beyond iron are made in large stars with slow neutron capture (s-process), followed by expulsion to space in gas ejections (see planetary nebulae). Elements heavier than iron may be made in neutron star mergers or supernovae after the r-process, involving a dense burst of neutrons and rapid capture by the element.