Why do mileposts on US Interstates increase South to North instead of North to South?

Comparing milepost numbering to the route numbering of the US Interstate Highway System is wholly unsatisfactory, as the practice of numbering from south to north predates the Interstates. At first I thought it was following the precedent of US 1, which famously has mile marker 0 in Key West. Then I thought maybe the tradition went back to the Boston Post Road, but then I realized that many of these early American roads (the National Road, Lincoln Highway, etc. in addition to the Boston Post Road) had literal milestones that marked the distance remaining to cities of general interest. So, for example, on the National Road traveling westward from Baltimore, one would see the miles counting down to Wheeling, WV; then Columbus, then Dayton, and so on. But eastward, they would count down to these same cities in reverse order, finally ending in Baltimore. So clearly this wasn't it.

Finally, I came across another website that made the point that mathematical graphs count up as one moves to the right on the X axis, and upward on the Y axis. Plotting this along most maps, one would see numbers increase as one moves to the north and east. So it seems that's the answer: Because highway engineers decided to follow the convention set by mathematicians.