Why 'inexact' and 'exact' doesn't check 'number?' first?

[Nala Ginrut]

It's weird to see that:
(exact? 'a) 
================err msg===============
ERROR: In procedure exact?:
ERROR: In procedure exact?: Wrong type argument in position 1: a
==================end=================

And I have to do this:
(define (fraction? obj)
  (and (number? obj) (inexact? obj)))


Why not 'exact?' and 'inexact?' doesn't check 'number?' first?