Because that’s the way that modern English grammar has evolved. Not all rules of grammar are vital to meaning, but the less-critical ones are still there. Plus no-one sat down and decided what sequence to put adjectives in; it’s “just” what emerged (and you could probably do a doctoral thesis on how/why the sequence is the way it is, and whether or not it matters; doubtless someone already has, come to that). But we all know and understand the rules – even if we don’t always know that we know them until someone points them out (or breaks them).
Harry Ritchie’s book “English for the Natives” is a good read on this and similar topics.
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