Traditional database engines were designed in the '80s and early '90s. At that time CPUs were much slower than they are today. Even worse was storage, hard drive head positioning time was enormous¹. And CPU (or, at most, a few single-core CPUs) was assumed to be infinitely fast in comparison to IOPS. Therefore, systems were designed to save IOPS as much as possible, while CPU overhead was considered a secondary optimization target.