I had a similar discussion with a non airline friend not too long ago. I was deadheading to Hong Kong and they are required to put me in Business or First Class (we still have both). He had the same question.

The majority of people in Biz/First have had the ticket purchased by their company. I'm no different, my company bought me a ticket too, the only difference is that we own the ride. At some point the product has to be protected and the seat can't be given to customers for a lower price. In this case, employees get the seat as that won't set a precedent of paying the cheap price and angling for an upgrade. It is a premium product for a reason.