A company that makes a 2xCR123, like Surefire, will not advertise using a 16650 li-ion instead, as the voltage doasnt match. The light will not run with full output and likely drop out of regulation. If a company makes a light intended for rechargeables in the first place, it will be 18650 sized, which can also accommodate 2xCR123 if need be, like the Surefire Duel fuel models.
16650s are used by people to replace cr123s in lights originally only deigned to take those primaries. All modern lights intended for rechargeable, will be bored for 18mm cells. A manufacturer will bore their light for 16mm cells exactly to prevent people from stuffing 18650s in there. I personally have a lot of Keeppower 16650s and use them in my Surefires. Some lights like the P2x Fury can indeed run on a li-ion with no downsides, but this is rare.
If you mean why doasnt anyone make a Lithium primary sized 16650, the answer is lower voltage.
