If the taxes are made public, then the thousands of little bits and pieces will be scrutinized by thousands of members of the media and public, rather than just one or two IRS agents.
Imagine, for example, that the tax forms claim that a certain restaurant in Trump Tower made a net profit of $150,000 last year. That might sound plausible to the IRS, but it might sound absurd to the restaurant's manager, who might have records showing they were making vastly more, or that they were actually losing money (suggesting that money was being laundered through the restaurant).