Q:

The reason for saying "5 out of 10" instead of "1 of 2"?

I'm just being curious, i have seen on some places, statistical results where they give them in form of "10 of 100.000" or "5 of every 10".
I don't have any statistical knowledge over my personal experience/reading around. But basic maths tell me that such values could be simplified as any fraction we used to do:
10 of 100.000 -> 1 of 10.000 // 5 of every 10 -> 1 of every 2.
Is there any reason not to do it?
Edit: 
The numbers come from, if i remember correctly:

10 of 100.000 people per year  with some bad reaction to medication
5 of every 10 people who actively smokes on ¿Spain?

A:

The procedures you describe are used in practice and in fact in many cases they are recommended for numerical stability of statistical estimates. Those procedures are called centering, standardization or normalization depending on procedure. 
Examples:

if your variable has some known "baseline" value, e.g. it ranges from 100 to 10.000, then you can subtract the minimum from it, so it to starts with 0,
if your variable is continuous then you can subtract the mean, so it is "centered" at 0,
with continuous variables you can subtract the mean and divide by the standard deviation, so to have values in so-called Z-scores (normalization),
you can subtract min and then divide by max - min to get values in [0, 1] range,
you can use all other kinds of transformations like squaring, square-rooting, taking logs etc.

In many cases such procedures are recommended. On another hand, taking the raw values could be helpful for making the results easier to interpret (e.g. age 45 is easier to interpret than age -5 if 50 was a "baseline" age in centered variable).
In some disciplines there are standard ways of describing some numerical values and your example is one of them, because in epidemiology it is common to report prevalence as $n$ cases per 100 000 people. It is a matter of convention.

