People severely underestimate how hard it is to build a head-mounted display that is small, light and provides good, high-res image. To do it cheaply adds another order of magnitude of difficulty. It has been the pursuit of many technology companies for decades.
I'm pretty sure, the Vuzix device is in the tens of thousands of dollars. And it's on a wire.
Oakley might hint all they want, as many companies have over the years, but they admit themselves in the article you've linked to that they've been chasing this unicorn since 1998.
And devices like the Epson Moverio BT-100 or Sony HMZ-T1 are a whole different class: they're entertainment clunkers that you wouldn't wear on your head on the street. What Google will likely offer is of unprecedented size and weight, bearable looks to wear among real people (not nerds), it's wireless, capable of linking with your device of choice (smartphones, possibly laptops), and will have to carry price tag acceptable for more affluent consumers (say, those who can afford iPhones and top-of-the line Android phones). So, when you ask, is it really that hard, I would respond: you have no friggin' idea.