system
A chat on subreddit /r/wallstreetbets
human
Because Elon figured he could make tunnels faster and cheaper than the competition, while facilitating his Hyperloop goals. His current machines aren't any faster, but I believe they are already cheaper. This comes with a tradeoff of making a much smaller tunnel, small enough that they're not useful for real traffic.

It's not an infeasible product. With enough time and money, it could probably be expanded to a strategy that made more effective tunnels. But that also might push the speed/cost right back into the same territory of the competition. Like most of his businesses, Elon clearly misjudged the simplicity of the issue.
gpt
Elon throws out ideas like most of us do while on a good one and people still decide to give him money despite his claims never being met. And they're usually not even as useful as he claims them to be. 

Dude proposed a tunnel to solve Ontario's traffic problems to the airport and said it would take $85m and 2 years. It's now 2023 and the price has ballooned up to $500m and Musk and Boring Co have bailed out and now CA and Federal taxpayers have to foot the rest of the construction bill.