Because a robot that does flips can also flip burgers, pick inventory, sneak up on people (i.e. military) and ultimately that stuff will be worth a lot.
One of their lesser goofy investments.
Google dumped the robots for strategic reasons, moreover, it's going to be a while before payoff.
They are making 'real' investments in late stage companies with actual business models, or things that have obvious potential upsides.
Softbank is basically where companies go for D and E rounds instead of going public. So there is some risk, but way less risk than left-field, early stage investments.
Their 'real estate' investment is in WeWork, which probably will make them a lot of money as long as interest rates don't flinch higher, and as long as there is no real-estate crash.
Sprint, ARM, Yahoo, Uber, Slack - these are not 'crazy' investments, especially depending on price. Yahoo might actually have some 'decent fundamentals' on some level, and be worth something at some price. They have a gigantic audience, and some small changes might make them be profitable at some level, and they could very well be worth something, at some price.
It's a cross between classic private equity and late stage venture capital, all of their investments make sense in that context.