Uhmm...I'm guessing that by keeping hair moist and in a bag, the hair cuticles are coated and protected from enduring friction with clothes that would cause
wear and tear, and prevented from drying which again would expose the cuticles making them vulnerable.
When hair cuticles are unprotected by a moisturizer, serum, baggy...any friction the strands experience leads to raised cuticles and that start to rip. Hence the reason people that don't bun or take extra care of their ends, find they have split ends. Since cuticles run along the entire length of the hair, I'm guessing that the more exposed to wear and tear they are, the more likely you are to go from lying flat
to being slightly raised:
then this:
and if cuticles continue to split away, then the cortex is no longer protected, meaning split ends are inevitable and your hair thins.
By bagging, you keep hair protected from the wear and tear.
That's my story and I'm sticking to it.
(Photographs borrowed from the The World of Hair - courtesy of P & G Hair Care Research Center)