Let me go through a scenario to give an example: how much was Nokia worth to Microsoft? Their idea that "due diligence" would uncover a value, and MS should signal the due diligence and the value with the pennies in the offer price doesn't reflect the truth of the situation that nobody knew or could know the value.
I'm not talking about a hindsight analysis after the Microsoft's mobile strategy failed; let's say it had succeeded and Microsoft was now equal to Android and iOS in a vicious 3 way competition: how much of that value would you attribute to the parts that came from Nokia? How much of the value would be due to MS's software? whose marketing muscle was it? Since it would now be a threeway competition, profits would be thinner, how much of that could be predicted with "due diligence"?
You don't know, because nobody knows; these types of intangibles are on the balance sheet as Goodwill because it's not possible to put a value on them except immediately post facto an acquisition. Due diligence is to uncover that what they are selling is what you think you buying, not the value you think you can extract from it in combination with your own assets, that's a secret you keep.