Unity isn’t only a tool. Unity is also an ingredient. It’s shipped with the product and is an integral part of what makes the product work. Most OEM deal out there also depend on usage.
You want to ship a product with Neo4j (or any other software developer) under the hood? Go make an OEM deal with Neo4j and I’ll bet you it is going to be some deal that will be proportional to the amount of usage your product is going to get. Which is only fair of course.
Your race car driver analogy makes no sense by the way. A developer makes a product and that product is shipped many times to a lot of people. You could think of Unity as a pizza bottom and a pizza oven. The developer puts stuff on top, bakes it in the oven and then it is shipped to people. The developer has to pay for the pizza bottom and the cost of the tool will be discounted. The developer charges a price such that after subtracting the cost of the pizza bottom there will be a nice profit. Profit and cost will be proportional to the amount of pizza’s eaten.