Why is that? Well, they eyeball highly classified information as part of their job, as part of their duties in protecting US interests and US national security.
A simple screenshot from a cell phone could be highly compromising.
Trump disregarded long standing protocols regarding classified material by stealing the material in the first place. He then deliberately played "keep-away" and then deliberately misrepresented that he had returned the requested classified material, when he clearly had not. Fact.
Some of those documents were the fruits of human intelligence sources -- meaning "assets" gathering intelligence in dangerous places. Trump's disregard for laws and protocols endangers those lives. It may mean that the US had to remove those assets ... and therefore, lose the value of the ongoing intelligence collection.