Is there some reason they have to make these have 300 or 500hp? Or is there nothing to be gained in terms of cost and weight from having, say, 90hp (like my completely functional Mighty Max had).

For the most part there is a floor with EVs where if you go small in battery, weight, and motor... You have a golf cart, not nearly enough range.

As you increase each of those, a larger motor will probably be more efficient for propelling a heavier load with a larger battery.

Because of the instant torque plus high speeds of an EV motor, it's not hard at all to have high HP figures.