I had the same thought in the past. My view is that the due diligence is used to verify the good standpoint of the company and most importantly raise any red flags, related to the deal itself. Those red flags might be issues that are not presented within the annual reports and issues that might as well differ according to the scope of the acquisition. For big corporations there is a point on your comment but history has shown that unfortunately not all annual reports correctly audited, so from a safe side a DD should be sufficient to justify the reports and public data as well.
