They were originally drawn with brushes, and the stroke direction was obvious. In handwriting, you're also supposed to use the same stroke order and direction. A font doesn't have that, so some fonts use differences in shape and thickness of the line to indicate the stroke direction.

So I'd say that the difference is mostly how your font chose to indicate the proper stroke direction.

I l 1

O 0

We have plenty of more ambiguous characters in our limited alphanumeric set and if special care was not given by don't designers it could look exactly the same.

Every written system has its own clues that readers learn to focus on.

In Japanese the number and direction of strokes is important but may translate to barely noticeable pixel differences that Westerners would dismiss as noise.

I think that this particular pair is confusing because in Japanese orientation is not supposed to be important, but is, in that case.

I and l looking exactly the same in some fonts is a different problem, really. You're not being asked to pick out a two pixel difference in slope (seriously, two pixels using pretty standard resolution). You use grammatical rules to tell the difference, which is annoying but not visually difficult. And numbers aren't getting freely mixed with letters in text.

And probably why no one cares, especially when Unicode was limited to 16-bit.