I'd never heard of Lydosyn (i.e. a drug which contains only carbidopa).

So then, back to my own question from above: are there any other reasons why there isn't a drug which contains just levodopa and entacapone?

I guess one possible reason might be that there just isn't a big enough market for such a drug. Maybe you can just about do all you'd want to do, using a combination of Sinemet, Comtan and Lydosyn.