Well the answer to that is to eat more fat.

I ate only keto for years and I'm getting back to it now so I have some experience to speak of.

You are correct, fruit is mostly sugar so no fruits. Some keto adherents allow the occasional handful of berries, but I found that just made me unreasonably hungry later on. Not everyone has this reaction, though.

There is plenty of fiber in above-ground vegetables. And even if there wasn't, it's not like eating only meat would kill you, humans evolved on the plains and/or jungles of Africa where meat was almost all that was easily available.

If you are talking very specifically about a ketogenic diet, then fruits actually do have too many carbs to maintain ketosis. In that case, it isn't some irrational fear, but reality.

Re fiber, A significant portion of the population (maybe a majority) doesn't need much fiber to use the bathroom. It seems like this need is a common situation that people assume is a universal truth. Further, fiber can lead to constipation for many people.

Fruits, yes, because it will kick you out of ketosis. Fiber, every keto adherent I knew would eat fiber in reasonably large quantities because keto often causes constipation, and a lot of the substitutes for things with "regular" carbs were high in fiber.