1. Garage is full of tight spots. If something "goes wrong", you have to lift your finger off the cell-phone (capacitive sensor, so its innately laggy to a few dozen milliseconds), that signal then has to be trasmitted over the internet, to the car which then applies the brakes.
2. Or... you could be in the car itself... and directly connected to the accelerator / braking system. No delays if something goes wrong.
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At least with a parking lot, you can argue that you didn't feel like walking (even if you need to keep the car within visual range). But in a garage + visual range, it means that you're giving up control for what amounts to a party trick. Its not like Electric vehicles give off fumes, so there's basically no disadvantage to getting in the car inside the garage instead.
It doesn't seem like its even safe for that though.