Not sure that I have a good answer for that other than possibly sales volume.

I guess the other answer is, technically they aren't. Sure, you can source a very high end bicycle tire and then the least expensive automotive tire and the bicycle tire is going to be more expensive. But, having just bought within the past year, 4 new tires for our family vehicle, the bill was over $1,000 for 4 Continental tires, not anything spectacular performance-wise, just a solid set of tires.

Schwalbe does produce a 700x28 (28-622) road tire that retails for $18US. I'm not sure you could find any reasonably worthwhile automotive tire for anything close to that.