Sorry, but is this a trick question?
If the cell cannot be used in other lights, and I cannot use other cells in the light, it greatly reduces flexibility. Modifications also means added cost, a cost tacked onto the customer. And if / when the cell dies, it suddenly renders the whole light useless.
I personally also don't like onboard charging on any light, as charging a battery inside a flashlight with a random voltage essentially makes it a hand grenade. I always - whenever possible - remove the cell and put it in a proper charger with voltage control. Onboard charging also means another point of failure, a feature that also means added cost, and as it's essentially a hole in the light, there is the increased risk of water ingress.
The better question would be, why do proprietary at all when it does not add any benefits?
Money (greed) is the obvious answer, just look at every single tool manufacturer. No tool battery can be used with any other brand, to lock the customers to their system.
EDIT:
I stopped reading after this, because now I know you're joking.
You know that's a fire hazard.
Also, I only charge my phone every Saturday.