These reasons have to be understood before any widespread change in this practice will occur. Actually, learning 'what to do' is sometimes much easier. If the goal of the mathematics instruction is to prepare students to product a page of correct answers, teaching the appropriate procedures will meet this goal more quickly and easily. The textbooks emphasize the learning of procedures. The pressure of tests looms.
Standardized tests are another reality.
Teachers are accountable for students' performance, and, in some communities, test scores are published in the local newspapers. For the most part, these tests evaluate students' proficiency with mathematical procedures. The procedures themselves are more quickly and efficiently learned by focusing specifically on how to do them. The fact that students continue to do poorly on test items that require more than rote learning does not seem to be compelling enough reasons to change the current emphasis on procedural mathematics. It is difficult to assess if students understand the 'why' of arithmetic.