From what I've been told, Intel allows 2 so you can go try to start a company. I once worked at a startup founded by a whole group from Intel doing this (including one of the original architects of 8086!). Several were on their last attempt. Every one of them went right back to intel, after it didn't work out.

And, from what I've been told about Apple, the idea is that if you can't find something within Apple (since they literally do everything, even textiles!), then Apple probably just isn't a good fit, especially since it is possible to spin up a team to make something new internally.