Because they would pass through states with a route 50/60 apparently. DoT mostly avoided this by using a mirror system with the existing US highways but obviously this leaves one problem area, the middle
The Interstate Highway System is a miracle of modern technology and the power of a functional government. The amount of technology in our roads is astounding. Asphalt itself is a wonder material. And we recycle 98% of it. (fuck you solar freaking roadways)
Odd number highways go north-south. Odd Highways ending in "5" are considered major arteries. Even numbered highways go east-west, and those ending in "0" are considered major highways. A three digit highway is a spur (First digit is odd) that only connects once to a highway and it is a loop if the first digit is even. The 405 is a loop that touches I-5 twice. I-880 is a shit ass portal to hell loop that touches I-80 twice, while I-105, for example goes from I-5 to LAX.
All made possible by Eisenhower who was ashamed of the sorry state of the US surface transportation network and awestruck by the German Autobahns. The best thing about the Highway act of 1956 is that it standardized all roads signs, widths and construction so that a car going from California to Boston, for example, could expect the same quality of road and understand the signs all the way across the country.
$500 billion total cost, 48,000 miles and who knows how much benefit to the US economy over the years. That and the vaccination programs started under Eisenhower are the best parts of his legacy.