Given the choice between having a sexual harasser on staff, or missing out on a billion dollars of profit, every company in the world will choose to keep the sexual harasser and the profit, so that can't be the only reason.

That's a very unrealistic perspective on management. To begin with, no individual is ever seen as personally irreplaceable.

Some are, but it's usually based on how much the CEO plays golf with them, not their talent. But in exceptional cases, like an acquihire, it might be. There's literally no point doing an acquihire and then firing the people you acquihired. That's just burning money.