1) LOL, if you believe less energy is consumed at higher RPM in an electric motor, I've got perpetual motion machine to sell to you. A transmission allows you to operate the car at highway speeds at a much lower engine RPM.
Yes, you can get slightly decreased energy usage at lower rpms because of lower friction and less slip on an induction motor (the kind used by Tesla), but throwing a multispeed transmission in there adds weight and complexity, and also increases drivetrain losses (transmissions are inherently lossy), though the amount of loss over a single-speed gearbox is probably not much.
As I've asked before, what other applications have an electric motor paired with a multi-speed transmission? Train locomotives like these? Dump trucks like this one? Ship propulsion units like these? No, these all have motors either directly driving their loads, or using a single-speed reduction gear. And trains at least have a much higher typical speed range than cars do (0-150+mph for Acela Express, 0-220mph or more for high-speed trains outside the US).