Good point, but many of them replace their old cases with the new versions.
The Antec series for instance. They phase out the old lineup when the new lineup comes in. And they all generally feature some sort of improvement or major design change that warrants having two similar cases.
Silverstone's Sugo series does tend to be a bit redundant - but they only have the 05/06 similar and 03/04 similar and there are some VERY clear differences.
You will still find old cases on store shelves while retailers clear out inventory - but rarely do the manufacturer's actively sell two of the same at once.
Lian-Li on the other hand has all the same products being sold at the exact same time, and many of them are listed as "New":
There mid tower section alone:
Lian Li Industrial Co., Ltd.
I checked out some other manufacturer's and found some other examples, Inwin like you mentioned. A common theme seemed to be that they were all overseas manufacturer's - maybe it is just the way they do things?