Only a tiny fraction of existing maths can be done with proof assistants currently, and as a result very very few papers use them. In most current research automated testing would be impossible or orders of magnitude more work; in many areas mathematicians are working with things centuries ahead of where proof assistants are up to, and working at a much higher level of abstraction. Also, many maths papers have important content that is not proofs (and many applied maths papers contain no proofs at all).