Do you know how ridiculously difficult it is to prove to an established business that "no, I don't have the specific experience you are looking for, but I can learn it in short order and I'll be one of your best employees," and then stand out from the 100 people behind you who say the exact same thing?
It's also ridiculously difficult for companies to figure out who the talented people actually are.
If you're really talented, what better way to prove it and lose nothing in the process? And what better way for a large company to get good people on board?