Are you serious? Do you really think thats the reason that this is happening -- that people don't just eat their veggies? Fiber is important but, um, that's a pretty hot take.

I suspect there are other factors at play.

The diet of most people in the US is pretty horrific. Absurd amounts of sugar, little to no vegetables, little fiber, lots of heavily processed foods.

That certainly does not help the situation. Whether it's correlational or causal I'd leave up to people more knowledgeable in the subject.

No health issue is so easily reducible, but the impact of eating a diet of "actual food" and moving around even a little bit daily cannot be overstated.

The odds ratios for nearly all diseases and all-cause mortality shift so far from those two interventions it's almost unbelievable.

We really don't know the reasons well. Food... some kind of bacteria or virus... 5 random things stuck together... it's silly to guess.