They are pointless for an electric car i agree.

For retrofit application it's easier to keep the gearbox and only use the 2nd or 3rd gear for exemple, you set the right gear and never touch it again. Like that you have an already made adapter for the driveshaft with the correct drive line angle, you have frame mount and support with factory and easy to find oem bushing, the gas engine was already in a good spot for weight management so the electric motor should be at the same placé etc. It's just easier to do it that way, and you limit the number of custom fabricated parts needed and cost.

For brand new car on the other hand there is no point to fit a gearbox and it's easier to use an entire axle with motor included like tesla's as you can design the frame to fit around it.

For low production volume it's easier/cheaper to use a close enought ratio from an already existing gearbox than designing a new one just for the engine/rear end combinaison.

Most people who does this kind of conversion themself tend to chose one good gear ratio and lock the gearbox to only use this gear without à clutch, the motor straight on the input shaft.