Because George Lucas was not a quantum physicist, nor all that heavily educated in astromechanics when he wrote the script to Star Wars (nowadays referred to as "Episode IV"), and because none of the many people who read the script before the film reached the theaters knew any better than he that a parsec, or paralax arc second, is the sixtieth of the sixtieth of a paralax arc degree, because most high school students don't learn that the three-hundred-and-sixty degrees in a circle can be divided into sixty minutes and even further into sixty seconds per minute, and that THAT is what is meant by seconds, not the sixtieth of a sixtieth of an hour.

That is why Han Solo said it.

HOWEVER................................

it COULD be argued that the speed of the millenium falcon, with Han's "special modifications" no longer operates within the realm of standard quantum physics.

The theory of warp drive is that you can generate a warp field so that you bend the very fabric of space (and time) and can take a trip that would otherwise take millions of steps in one single step.

The kinds of modifications on the Millenium Falcon, therefore, could conceivably behave in some other way.

Does anybody actually say exactly how many distance in miles or kilometers this "Kessel Run" is?

It could easily be that the millenium falcon made the Kessel run in the same amount of time as any other ship could have travelled twelve parsecs in a warp field.

Or it could be that the kessel run is normally a run that would require a ship to travel hundreds of parcsecs but Han's modifications made it so that the warp necessary would only be a leap of twelve parcsecs.

It's called science fiction for a reason: it's fiction, not a bit of it is real.