It's not about learning Fortran being hard, obviously if you're good you can acquire any language, especially in a paradigm you already know anyway, but why ?

That's the thing for these "top-rate" posts, you need to sell the candidates on why they want to take your offer, not some other role they could have. Yes, one option is to "just" pay them a tremendous amount of money, but remember you're competing with the banks who have the same strategy and their whole business is making money. So that would be very expensive.

Instead it makes sense to compete on other considerations. Candidate doesn't like to work in the office? No problem. Stay where you are, work for us remotely. Or maybe they'd like to move their grand parents and extended family to your country? You could make that happen. Or could you? Do you think the US Federal Government will accept "We need to naturalize these sixteen random people, immediately to get one Fortran programmer" ?

And the choice of language is one of those other considerations. To a first approximation nobody is more keen to take your offer because they get to write Fortran.