Doctors have an incentive to order as many tests as possible

This would conceivably be true for car repair as well. A mechanic is incentivized to order as many repairs as possible for a car.

So why don’t they?

The answer is many-faceted, but the main ones are (a) professional ethics, (b) reputation, and © second opinions which kinda feed into b.

Any provider whether doctor/mechanic/wedding photographer/whatever is also incentivized to serve their customers well by selling them only things that truly benefit them.

We don’t need insurance companies in all those other industries to prevent providers from using an infinite-billing hack to generate infinite money.