That’s part of it. The fuel economy is not better with a manual nowadays. (Or marginally so.) And many never learn how to drive one, because even the economy cars sold here usually have an auto except on the very bargain-basement trims.
Reality. People want hassle-free cars that are fun to drive and have creature comforts. A person buying a car to have for 3 or 5 years doesn’t care if the transmission will last 175k miles.
If you want that to change, don’t blame the car makers. Convince the American car-buying public that they should insist on no-frills cars with manual transmissions so the guy that ends up with their car 10 years later has an easier time.
In many areas of the country, the car will rust long before you get to 175k miles. The car/truck market is not composed only of people who want to buy a car and keep it for 20 years until it falls apart. That’s a small segment of the market.
Car manufacturers sell what people want to buy. For most, the benefits of an auto transmission far outweigh considerations of what the transmission will do after 175k miles.
The dealers order the trucks. Not the public. What the dealers want and what the public wants arent always the same. If more manuals were offered more would be sold. If they ordered 20% manuals they would have all been sold.