One problem is that explanation is in-and-of-itself a separate skill, and is, in many cases, harder than math (at least K-8 math). So asking for an explanation in addition to doing the math is asking for 3x effort. Moreover, explanation is highly ambiguous (whereas math ain’t!) So you’ll often get an explanation that you don’t quite understand bcs the kid isn’t good at explanation, even if they know perfectly well what they did.