Why Musk would spend probably tens of millions of dollars on this prototype which clearly will never be drivable on roads. One explanation is he is a trillionaire and the 14 year old in him thinks it would simply be cool to do it.
I think there is a more rational explanation. Tesla sales have slowed, other companies are catching up, and if it weren't for import issues, Chinese cars would be eating Tesla's breakfast, lunch and dinner. Yet Tesla is trading at a 330 P/E and Musk has to do something to make it seem like Tesla is still light-years ahead of everyone else and just wait, that 330 P/E will be justified any quarter now. Just like shooting a roadster into deep space was just a PR stunt, this is a PR stunt to boost the brand with no actual market relevance.