That's like saying the airbags and computer skid control should have been in cars since the beginning in 1901.
Weak analogy. Apple has had multiple monitor support for years. There's absolutely no excuse for them not having full support for multiple monitors from day one of adding full screen mode. It's not an issue of technical innovation, it's something they were certainly capable of doing at the time but just didn't get it in. And a couple years later, it still isn't in.