> I don't understand why they can't at least give solid reasons
Probably the liability around someone saying something not accurate, or something that could be construed as approaching, even indirectly, some protected class. They've already laid her off, she's not going to be singing their praises under any circumstances, anything they do at this point just increases their liability.
In a perfect world I'd love for people laid off or fired to get detailed reasons why so they can adjust course if necessary or at least know that it was nothing they did and just budgetary, but there are too many people who will want to argue the point in court that the company not only has no incentive to do it, they're heavily incentivized not to.