Certain things the way they are .......????
For Example.......The US standard railroad gauge (width between the rails) is 4 feet, 8.5 inches. Now that is a very odd number. Why was that gauge used? Because that’s the gauge used in England and the first US railroads were built by English engineers.
Why did the English use that gauge? Well, the first railways were built by the same people who built the pre-railroad tramways, and that’s the gauge they used.
Why did they use that gauge? It’s simple, the people who built the tramways used the same jogs and tools that they used for building wagons which used that wheel spacing.
Okay, so why did the wagons have that odd wheel spacing? If they tried to use any other spacing the wheels would break on some of the old, long distance roads in England because that was the spacing of the wheel ruts.
Why were the ruts that wide? The first long distance roads in England (and most of Europe) were built by the Roman Legions and these roads have been used ever since then. And the ruts in the roads? Roman chariots first formed the ruts and everyone else had to match the ruts for fear of destroying their wagon wheels. Since the chariots were made by Imperial Rome, they were all alike in the matter of wheel spacing.
Consequently, the United States standard railroad gauge is derived from the specifications of the original Roman war chariot. Yes, bureaucracy lasts forever!
By the way, why was the wheel spacing of the Roman war chariot that width? It was just wide enough to accommodate the back ends of two Roman war horses.
Thus, we have the answer to the original question. Now here is a modern twist to the story…
When you see a Space Shuttle sitting on its launch pad, there are two booster rockets attached to each side of the main fuel tank. These are the solid rocket boosters (SRB) that were made by Thiokol in their factory in Utah.
The engineers who designed the solid rocket boosters might have preferred to make them a bit wider, but the SRBs had to be shipped by train from the factory to the launch site. The rail line had to run through a tunnel in the mountains. The SRBs had to fit through that tunnel and the tunnel was only slightly wider that the railroad track, which was slightly wider than two horses behinds.
So, the major design factor of what is arguably the world’s most advanced transportation system was determined over two thousand years ago by the width of a horse's ass.