Most (higher end) battery-powered bicycle lights have built-in Li-Ion battery or a battery pack nowadays. There seem to be no front lights using replaceable 18650 cells. Rear lights with replaceable batteries are widely available in my area (EU).
Li-Ion batteries have limited life and as such they limit lifetime of the whole light. If used everyday, the capacity is gone in one year. With replaceable batteries I can carry spare charged units for long night rides. 18650 batteries are widely available and seem to be natural "successors" of NiMh AA batteries for applications like bike lights, but such "evolution" didn't happen.
Is there any technical (not marketing) reason for this?
(I also use dynamo lights. This question is specifically about 18650 batteries.)