Its because electric motors are so efficient across the whole range of normal speeds with a single gear ratio that its not worth the extra supply chain complexity. In terms of range when comparing the gains from the efficiency with the multi gear transmission vs the additional stored energy of the same weight in batteries, the additional batteries wins every time. Its a fraction of a watt hour a mile gain, vs about 10 additional kWh which is another 40 miles of range at highway speeds.
The only reason with an EV you need multiple gear ratios, is to keep the motors in their power band at higher speeds, and its possible to achieve that just by having different motors with different ratios.
The reason resto mods, or the Ford Mustang electric concept car, or this Dodge concept have manual transmissions, is that's the simplest way to adapt an internal combustion vehicle to electric, but it does make it difficult to pack in more than about 30 or 40 kWh good for about 100 miles of range.
I suspect this is like the Mustang Lithium, a concept car they'll trot out to some dragstrips to make headlines, but not something they'll actually produce or sell.