Heroes 5 mini

I don't understand one thing: why do people run away so often, even with a starting army that costs only 5,000?

First of all, sometimes it's just stupid to fight... when I first started, I was annoyed by the naivety of the opponents... I have a basic army for 5,000, and they have an army for 50-80 thousand... :( So, what's the point of fighting...

update
and another question: how is the number of kills calculated? based on the stats in Heroes of Might and Magic 5? or according to some other formulas?

Some kind of their own formulas... For example, Necropolis has dragons at very cheap prices, but if they don't hit very hard in Heroes of Might and Magic 5, then here they are a full-fledged level 7 unit :( (FOR HALF THE PRICE).