Why don't LEGO Minifigures have ears?
I can not give you an answer on behalf of TLG, but I can give you some good reasons as to why Minifigures do not have ears.
Authenticity: Lego figures have never had ears. In fact, the first figures did not even have faces, or articulating limbs. They were very basic. Here is a photo showing the basic evolution of minifigures over the years. The first 'figures' were introduced in the mid-1900s.

Functionability: Adding ears to the head mold would greatly complicate things for Lego. Apart from the fact that ears would be likely to be broken off or get damaged, they would affect the compatability of heads.
All standard mold lego heads can fit just about every mold of hat and hair. However, if TLG started making heads with ears, they would have to redesign every single 'headgear' and 'hair' mold so that they would fit around/over the ears. As a result, for figures where ears are necessary, they are part of the headgear mold, not the head. You can see this in these two examples.


I hope that answers your son's question, and that you can simplify the reasons for him :)