tl;dr: because we believe it's a gimmick without much utility.
The proponents of assigning a score claim that there's a difference between "best food ever eaten" and "good meal". It might be the case if we just look at one particular review, however:
The gradation of the superlatives is highly subjective. Is "wonderful" better than "excellent"? Is "excellent" the same as "great"? Is "good" the same as "pretty good"?
The choice of the adjectives is often influenced by the background of the reviewer. British "could be improved" may mean "horrible" in another culture, and vice versa. The background is more granular than a country: it may be a region or the person's upbringing.
With multiple aspects, the results become much harder to read, integrate, and aggregate properly.
We believe that increasing the variety of aspects is better aligned with the actual business needs.