investing means some ROI. There is no way to have any ROI here (to illustrate - try to imagine how you can get any ROI by investing back at the time in Einstein developing GR). They aren't going to build a lot of power plants themselves and for selling their tech - there should be some "secret sauce", and the current fusion development is at the stage when results of one team can easily be replicated by any other team (given sufficient access to money). I.e. current investing in fusion is more like scientific/humanitarian grants (and big thanks to those guys like Allen for pushing our civilization forward) than VC for profit.
For example, once a team develops high-performance sensors and software to efficiently manage say that plasma ball at the end of a DPF shot so that there would be a real power gain, that would be an item suitable for investing by VC.