"Mandatory" was not the best word to use. Short of plagiarism, libel, slander and the like, reviewers are free to write in any fashion they wish. If they write in a manner that readers like, they'll be rewarded with a devoted following and continued work. If their work is meaningless to the majority of readers, they'll find themselves looking for a different line of work. And, lots of reviewers are in the middle, with some readers loving their work, other not, and a healthy contingent in the middle who are "OK" but maybe not thrilled.
If one is a long time reader of a magazine, you'll have your own mental list of reviewers to which you pay close attention and others which you ignore or discount. And, that's not to say the ones you like shouldn't be open to suggestions to improve, though as several have noted above in this thread, sometimes practicalities get in the way. How often, for example, does the reviewer have the prior model sitting side-by-side with the new one under review? If not, that means that any comparison would be from memory, and probably have been in a system with completely different companion components. Such a comparison would make for interesting banter, but hardly a solid reference point.