Why setq/set! is a Special Form?

What stops use from replacing setq/set! with explicit memory cells? That would at least make it a function instead of a special form and compiler could still easily inline it.

I.e. a for loop would look like:

(let (I (cell 0))
  (while (< I 10)
    (print I)
    (I 'now (+ I 1))))

>>1
Moreover, such approach gives us ability to pass such `cell` as an argument. While you can't take a pointer to a usual variable, cells are pointers themselves.

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