Generally speaking, this comes from a number of different factors, both technical and psychological.
For one, early RPGs had technical limitations imposed on them as to the number of differentiable values, especially in the era of 8-bit and 16-bit processors in game consoles. The maximum range of 8-bit integer values is 0-255, and for 16-bit integers, 0-65535. (In addition, I won't even go into how difficult it would have been on early platforms to represent decimal values.) This left a very limited range of relevant values for game designers to work with, so they (mostly) chose ranges which allowed decent levels of differentiation and gameplay balance, allowing for probabilistic and stat-based strategies to be created (think Pokemon's damage system), while still being within the ranges allowed by the processor.
In the above comment, the concept of aesthetically-pleasing numeric values is touched upon. These numbers are easy for us as humans to grasp, since they avoid what I like to call "zero shock" and "decimal/fraction shock". They are integers, which means we don't have to deal with differing scales or having to interpret decimals mentally, but they don't have an excess of wasteful information (extra zeros), which is not relevant to quantifying the importance of the value itself.