Why? You are already getting a loot from the war and the new war chest is added too.
And yeah, we need to raid more times to fill our hero chest as we can’t expect to win all our raids.
If you find it difficult to kill the enemy heroes, then spend some time reading about them and what your own team/heroes can counter with, for example dispelling, healing and so on. And also remember that the order that you are firing of your heroes also has a big effect on the end result of your attack.
Boldtusk - Gormek - Marjana = high damage
Marjana - Gormek - Boldtusk = low damage