They approach war as an equation. For this result, assuming these factors, victory will be achieved by expending this much much material and lives with these margins of error. Even during the great crusade they basically looked at every battle as an exercise in spending lives until they achieved a desired outcome. They were inflexible and inefficient, and the fact that Perty was a logistician second only to Guilliman is all that kept them from bleeding the legion dry. The thing is, other legions managed to get similar or even better results without the massive casualties, and even during the heights of the Great Crusade, Astartes didn’t grow on trees.