The problem is that the grading criteria is too subjective. It should involve easily measureable and therefore repeatable items like centering, corner roundness, gloss, coloring, etc. Words like "sharp", "minor", "slight", "most", etc. are too ambiguous. PSA should give a measured explanation like 58/42 centering, one corner rounded 12o, 78% glossiness, 94% color saturation. Obviously there are other factors like stains, writing, nicks, etc. Even those could be measured by some criteria. The score factors could be revealed without giving the actual formula for the final grade. How many episodes of Deal or No Deal aired, and I don't think anyone ever figured out the banker's algorithm. Even with that many examples, there were simply too many unknowns to give away the formula, so PSA's intellectual property would be protected.
Personally, PSA grading doesn't matter to me. When I see a ridiculous price for a PSA 8 or 9 on eBay with the wording, "none graded higher," that's pretty meaningless. There are probably 100 or more that will never be sent in for grading for every one that was. And what about the cards/stickers in unopened packs. Isn't it like Schrodinger's cat? There could there be a PSA 10 in any given pack, but we'll never know unless we open them.