Due to the 3 successes per dice for attacker vs 2 successes for defender (w/o cards).
For example in a siege with 5 attacking armies vs. 4 defending on a 3-power province with a fort, a chance to win as a defender is just 16% (averaged from 100k combat simulations).
5 attackers vs 5 defenders on a 4 province (maximum possible defence), gives a 30% chance for the defender, and 26% if there is no fort.
What do you think?