Microsoft, Google, and Amazon are interested in it enough to put money into it. That suggests there is a market need for it, or at least interest, within the current context.

We should assume they're acting rationally, so the real question is, why do they find this interesting at all? Why not dump the money into private solar farms instead?

The world will spend about a quadrillion dollars on energy in the 21st century. Piddling little billion dollar investments look like long shot bets on low probability outcomes. The big spending is on renewables now.

Gates in particular seems to have been a disciple of Vaclav Smil, a person whose arguments against renewable cost reduction were wildly mistaken.