Because most clans like to minimum hit raid Titans and even normal Titans to get maximum resources out of it. It would cut down time if we could toggle Minimum and maximum damage of monsters with your highest tier troops. Also would help get more points in All You Can Slay events.
Why not have it so you tap the little icon above the march strength to have the march strength set to the minimum required to hit the target?
Believe me. ATA is perfectly aware of everyone's desire to have this feature implemented. It will almost definitely happen... It's just a matter of time.
As 3cmrgx3 has already stated, this idea has already been posted numerous times. Instead of creating new topics, please read previously posted topics and support the idea which has already been posted.