a coin's grade is generally determined by five criteria: strike, preservation, luster, color, and attractiveness. you could argue that plastic could be graded under preservation, however plastic coins are not created by dye stikes, have no luster or the colour and attractiveness of metal.
I would agree with your 1st 3 points but as far as luster, color, and attractiveness of plastic compared to metal, this would depend on manufacturing and techniques involved.
Metal coins are harder to make/strike (esp like Platinum), prone to coloring issues, easier to scratch/dent etc. Plastic is far less subjected to these issues and when making in batches you’re likely to get all perfect matches. Metal coins have stuff like cameo etc and a batch of coins can very quite a bit. So why grade plastic that really is nothing like metal? Doesn’t make a lick of sense to me.
To answer your question, because it's a collectible. This is why we send things off to be graded, regardless of the medium.
Baseball cards, metal coins, comics, ticket stubs..those are easy to damage and hold much more clear grading criteria. A plastic satori, unless you loosely carry it in your pocket with your keys and change or whatever, for a week, is always going to get a high grade because they are easy to make pretty flawlessly and hard to fuck up. VERY FEW people are going to care about a poker chip grade, and would be just as happy with it slabbed and authenticated. My point is you're in the very minute minority with this. Nothing wrong with that, and if it's what you prefer, than hey that's all that matters. I respect your opinion though and we can agree to disagree of course. But we should prob both agree to stop shitting all over this thread now
(sorry OP)