Because it’s somewhat easier than 72. The logarithmic calculation applied to compound interest. 
You can use 70, 72 and 69.3.
I use 70 for ease of calculation.

Rothbardian_fanatic

jk_14

It woks as doubling if you are saving.
Divide 70 by the rate of interest to find out how long it takes your money to double.

I don’t think the value of your money halving in value is good. The other half goes to THEM.

denlillaapan

econ

Central banks need escape route from cycle of boom and bust

south_korea_ln

Let’s see, 2%, OK. 70 divided by 2 equals 35. So, at 2% your money’s worth shrinks in half in 35 years.
Yeah, good deal!!!!
If you are saving for a retirement, this halves your money every 35 years. 
Save now halve later!!!
It also raises the interest you require to increase your savings by 2%.
I don’t see that happening at the zero bound we were in and returning to.
Look to Japan to see the results of the zero bounds interest rate.
It is coming here sooner than later, unless we get rid of the Fed.