There's a lot of extra hardware that goes into a robotaxi. Waymo doesn't just sprinkle pixie dust on them to make them drive themselves. The vehicles, their sensors, compute, adn the 100 miles of wiring needed to hook it all up is just hardware costs. There's all sorts of logistics and real estate that needs to be acquired. Every new area needs to be mapped and tested. They run call centres, and pay thousands of people just to do data annotation.
The plan for all these companies is to scale up to deployments of millions of vehicles, they won't see ROI for a decade or more, and that's only if everything works like it's supposed to, it's still technically and unproven business model built on an unproven technology.
And sure GM can raise funds if they really want to, but they've gone from 0 to $60b in debt in the last 6 years, with $20 accumulated just over the last year. They have to deal with converting their fleet to electric over the next decade. Very expensive. Lots of risk. GM needs to pick and choose which risks are worth taking.