Why?
Understand that Speed is Distance over Time. Time is the culprit here because there are 60 minutes to an hour, not 100.
Imagine the complexity if you child starts putting in the decimal answers in his workings. He'll likely have to write down a very long chain of decimal points.
For e.g., 3km ÷ 8min. Would it be easier to pen down as:
a. 3 ÷ 8/60, or
b. 3 ÷ 0.133333333 ?
Source: Mendaki.org
Experiment #7 - Forces (Frictional Force)
Experiment #6 - Forces (Frictional Force)