Because silly, thats not where the revenue is going to be generated. Increased demand means more car sales, which means more car production, which means more people working and earning wages to make and sell cars and more people earning wages manufacturing and selling parts, materials and services to car manufacturers and the people making the money from increased manufacture and sales and service generating more personal income and paying taxes on that additional income and spending more of that additional income on consumer purchases (which are taxed) and on and on. You're just not doing the math.