Keep this to yourself, but it's perfectly possible to play Mario Kart Live: Home Circuit outside. Nintendo doesn't want you to know this - in fact, the game's packaging warns against doing so - but you can, if you so wish, drive your little RC car outdoors.
Because the connection exists between the Switch console and the car itself (there's no internet connection required), you can literally take it anywhere you like. Woo-hoo, right?
However, as our mum used to tell us many years ago, just because you can do something doesn't mean you should. The cars aren't designed to be used outside and mud and moisture will most likely have a detrimental effect on the car's performance - which is no doubt why Nintendo advises against it.
Still, it's fun to not do as you're told, right?