They idolize victimhood, and getting sick from the single most common food lets them pretend they have a disability.
It also allows them to harass low-wage employees. Service workers didn't pick "Karen" as their go-to stereotype by coincidence, it was always a certain demographic that gave them the most trouble. Now she can demand to know the full ingredient list, and waste everyone's time reordering a dish because the sauce had 1% wheat flour in it.
Accusing staff, or your hosts of messing up allows you to guilt trip them. Since they basically killed her, don't they OWE her? A free meal from the restaurant, or maybe a friend needs to make it up to her later.
It's impossible to prove a Karen is lying, so she doesn't have to worry about getting caught.
(Edited 7 minutes later.)
@previous (B)
Have you tried just not cooking shitty food?