Because if they did not, then they would not be number lines!
There are infinitely many numbers between any two different numbers. To find a number exactly in the middle between two numbers, calculate the average. That is, add the two numbers, then divide the result by 2.
Even numbers can be matched up in two lines. Odd numbers have one left over.
Numbers were named after the number of angles they represented, and each angle represented a quantity. For example, the number one has one angle, number two two and so on. They have to be written with straight lines (not curved).
If there are 4 lines between 3.1 and 3.2 on a number line, the intervals would be evenly spaced. The numbers between 3.1 and 3.2 would be 3.11, 3.12, 3.13, and 3.14. Each number represents an increment of 0.01.
Coordinate planes