This is a straightforward way of electric conversion. You plug in the motor in the place where torque is expected and let the rest do its job.

Gearboxes in EV conversions are usually locked in the highest gear with clutch and lever removed.

Presumably so it's an easier conversion - you replace the motor but don't have to replace the rest of the drivetrain, and maybe you want the gearstick inside for the look of the thing (although I imagine you likely wouldn't have to use it most of the time).

Probably to interface with existing cars by replacing the engine, retaining everything after, including the existing transmission, differential(s), and suspension.

It says:

"The current eCrate kit requires a GM 4-speed automatic transmission with an external mode switch (e.g., 4L60, 4L65, 4L70, 4L75 Transmissions). This helps to make the eCrate conversion easier for vehicles that already have a conventional driveline, plus it provides extra torque in lower gears and extra speed with overdrive. We are working on bringing a direct drive variant option to our eCrate portfolio."

Reduction ratios, greater comparability due to interfacing links, attachment points, just a few guesses. Most ev conversions I've seen keep the gearbox.