Because exact recordings are better than human memory. The pilot will be subconsciously changing their own memories slightly to fit a better narrative. This is

Because exact recordings are better than human memory. The pilot will be subconsciously changing their own memories slightly to fit a better narrative. This is just what human memory does and you can't turn it off. These changes aren't particularly egregious unless you are consciously lying, but the legal definition of perjury does not care. With written notes, those are going to be summarized in a way that adds doubt to any small discrepancies that might result from a pilot misremembering things. An exact recording makes it 100% clear that the pilot is doing that, which could be turned into a legal argument against them.