It's cheaper for a company to use in-house "off-the-shelf" parts instead of having multiple, specifically tooled assembly lines for each and every model of faucet. The "internal parts" are all likely to be the same.
The extra manufacturing cost (we have to ignore whatever marketing says, and the effect of supply and demand*) comes from the fact that plastic can be cheaply injection molded (once you have the die), and that metal casting still requires numerous additional steps in the casting and finishing processes.
Every piece of cast metal you've ever held has been made from a (single use) mold, which was made off of a pattern (which can also be quite expensive).
Any casting with internal passages also requires the use of cores; an additional expense. Again, each of these are a single use item and it's quite probable that it requires more than one per casting.
All-metal faucets cost more because they... cost more; in materials and labor.
*I'm unqualified to provide any type of market analysis on this matter.