The question’s pretty self explanitory. Why isn’t there a second word to complete the coupling?
More/less?
More/fewer?
Anyway, that’s my answer; more or less. 
How about greater?
We have more/less uncountable stuff, and more/fewer countable things.
If you need more than “more”, how about “a greater amount of” uncountable stuff, and “a greater number of” countable things…
Enough?
The ultimate answer is because language does not grow under any sort of logical plan. People felt that the difference between less and fewer was important, but that when discussion more items, it didn’t matter.
That’s how language works. It’s foolish to expect consistency.
As the great Madonna once said, “More is better than nothing, yes, but nothing’s better than more.”