tl;dr: Because there were ongoing investigations (which was true) and it's generally considered bad to release your evidence before trial, or something like that, IANAL.
This will also be Trump's (false) reason for not releasing them.
IMO the most egregious reason is the July 2025 memo from DOJ/FBI saying there was nobody else to investigate, after months of public interest and official statements they were working on it. If they now flop back to claiming they can’t release because of investigations, then that’s unequivocally a false reason.
Matt Levine has a common refrain which is that basically everything is securities fraud. If you were an investor who invested on the premise that Meta was good at AI and Zuck knowingly put out a bad model, is that securities fraud? Matt Levine will probably argue that it could be in a future edition of Money Stuff (his very good newsletter).
The "everything is securities fraud" meme is really unfortunate, not quite as bad as the "fiduciary duty means execs have to chase short-term profit" myth, but still harmful.
It's only because lying ("puffery") about everything has become the norm in corporate America that indeed, almost all listed companies commit securities fraud. If they'd go back to being honest businessmen, no more securities fraud. Just stop claiming things that aren't true. This is a very real option they could take. If they don't, then they're willingly and knowingly commiting securities fraud. But the meme makes it sound to people as if it's unavoidable, when it's anything but.
If Mark, both through Meta and through his own resources, has the capital to hire and retain the best AI researchers / teams, and claims he's doing so, but puts out a model that sucks, he's liable. It's probably not directly fraud, but if he claims he's trying to compete with Google or Microsoft or Apple or whoever, yet doesn't adequately deploy a comparable amount of resources, capital, people, whatever, and doesn't explain why, it could (stretch) be securities fraud....I think.
As a student in my forties I have been appalled at the price and quality of many of my textbooks. The $300 dollars worth of textbooks in one class could have done with a few youtube videos. The publishers know this cash cow is coming to an end due to piracy and online textbook rentals. Now, they are charging less for textbook but gouging students on the mandatory online components.
Summers is just weather vane for current economic thinking. He's not a particularly brilliant at anything.