Not to mention that they would be paying for an immense and unambiguously good scientific gift to posterity that would be free of all the complications, interest conflicts, mismanagements and so forth that come with building a mega-charity like the Gates Foundation (the conspiracy theories around that alone have been absurd, despite all its excellent health programs for the developing world).
It's always hard for a person to say what they'd really do if they were in other shoes, but if I had the sort of fortune that a Bezos or Zuckerberg has and especially with the relative youth of these guys, it wouldn't be hard to convince myself to make such an investment.
The Large Hadron Collider cost somewhere in the neighborhood of $10 billion dollars. Bezos alone saw his net worth grow by a multiple of that in 2020 alone. This huge cost would barely dent anything else they're doing or funding and in no way dent their personal quality of life.
I'm not at all arguing that they owe something like this to the world, or trying to paint any kind of anti 1% resentment, just pointing out that paying for this doesn't even seem like such a bad idea for people like them and their image for posterity (something always important to most billionaires).
The top four or five billionaires in the world could even cover most of ITER's much larger budget without permanently or seriously denting their fortunes.