A thought concerning the BLI stock cars: If I understood correctly all their car bodies are ABS, and they seem to be rather "solid" (trying to avoid the term "thick"...). Just remember the brakewheels. So even if the stock cars would have been see-through it might have looked strange as well. Some etched sides would have been more realistic, but probably not so easy to manufacture.
I have a 4-set on order, so I will see what comes out.