Why not sell individual pieces of rolling stock?

So I know there are exceptions to this, most notably the “My Own Train” line, but to me, it always seems like Lego is averse to selling individual pieces of rolling stock. For example, Emerald Night came with one passenger car, and if you wanted more, you had to either build them from scratch, or buy another set. Same applies to a lot of their freight sets, why not sell extra freight cars separately as well? Anyone else notice this? Any ideas as to why this is? submitted by ...