Consider SpaceX, its a growing business that took what, 10 billion in capital? That is below the 'material' threshold for some of these companies holding 100 to 300 billion dollars in cash equivalents.
Wouldn't that be too late? Or, put it this way: it is already a problem of capital. If you have enough, you can explore all technology tree branches till finding the one that leads you to it, even in parallel.
If there were a global carbon tax, it would give fusion an advantage against fossil fuels, but that would also be to the advantage of fission and renewables. In the absence of a global carbon tax, is fusion going to be cheap enough to displace fossil fueled power? We've had the technology for emissions-free electricity for decades, but it's difficult to profitably compete with fossil-based generators enjoying unpriced externalities.
They already are, in the cheapest, riskless way possible: those massive cash stashes are sustaining, or even boosting, their valuations.
When a company grows too big, it's in its best interest, as in investors best interest, not to spend a few billions on an initiative that could lead nowhere.