TL;DNR - single finger levers have little (if any) advanatage and some disadvantages.
Most people do not start out cycling with XT/Ultegra quality bikes and therefore (do not) start out on bikes that have brakes good enough for one finger control. Even today, many low end disc brakes cannot reach full braking potential (i.e. wheel lock), with one finger. Also think of kids' bikes and kids' hand/finger strength. No way an average child with an average child's bike can brake with one finger.
People transitioning from those brakes would likely be uncomfortable with single finger levers during the transition, and would likely not buy a bike equipped with them. If the bike might be resold or lent out, one with single finger levers would have a smaller potential market.
Poor maintenance, accepting pad contamination can occur at any time, means even the very best brakes may require more force than a single finger to achieve full stopping potential.
Presuming the bike has good, well maintained brakes and single finger braking is how the rider rides all the time, what's the disadvantage of current lever designs? They work very well on a single finger. A rider can intuitively feel exactly where on the lever the finger is to achieve maximum performance. Changing the shape will not improve braking, and there is no weight saving to be made.
The only advantage would be aesthetics and marketing spin to sell the new shape as a premium product aimed at the cashed up MAMILs.