> Why exactly can not it be filled?

Because eventually you’ll run out of people that can be hired.

Unless you’re making an argument that there are more software developers than job openings in the US, which is patently false.

And more importantly, this case is more about people who were already employed at Apple, on a non-immigrant visa, that Apple wished to retain permanently. I see no actual wrongdoing here. A tried and tested employee is ALWAYS better than an unknown one.