Why Not Have 30 Kilometer Running Races?

While I was contemplating which of the three offered races I would run in this weekend...along with the multiple distances for two of them...it once again occurred to me that there is a substantial, significant gap between two of the distances.  The so-called "classic" running races are 5K (3.1 miles), 10K (6.2 miles), 15K (9.3 miles), half-marathon (13.1 miles) and marathon (26.2 miles).  I've covered all of these in races since I returned to running in my early fifties, but that huge gap separating half from full marathons is very imposing: 13.1 miles!  Yet looking at it logically, there could be another distance offered that gives people the chance to either work themselves up to that marathon distance or find their own most suitable distance.  And that is the 30K race, 18.6 miles long and a fitting intermediary between the half and full marathons.  I wonder why this distance isn't ever offered...I think it's a shame.  I certainly would be one of the ones lining up to run in one of these..