Whats the deal with exercise bikes ?

I think it's because it's electronic. With electronic anything, especially cardio machines, there usually comes options such as calculating calories according to your age, weight and height, levels of resistance, patterns of resistance, distance traveled, heart rate, etc... You wouldn't be able to do any of that with a regular bicycle (except for resistance), at least to my knowledge.

I don't know how much stationary bikes are, but I'm betting it takes maybe $10 to make and of course the company is going to hike up the prices, like every other company does. It's how they keep a positive revenue.