Do we actually need gut bacteria? Do they serve other purposes that breaking down fibers?
We also know that antibiotics that kill gut flora lead to health/digestion issues until the gut flora is replenished.
It doesn't take nearly as many why questions to get to a "we don't know" answer with biology versus physics. There's lots of stuff about biology we can observe but do not understand the why as well as we understand something like the physics behind a microprocessor.
There are even theories that gut bacteria can drive human behavior at some level. https://www.theatlantic.com/health/archive/2015/06/gut-bacte...