Why are thumbs.db files stored within the directory they refer to?

Question

I would have imagined that - being nothing more than a cache of thumbnails - thumbs.db files would have been stored in some dedicated cache area of the Windows filesystem: perhaps Explorer's app cache or similar. It seems weird that this would be one of the very few instances where a cache in Windows is stored with the actual data itself. And why a file, rather than entries in a database?

I can only think of one possible reason: for massive directories of high-resolution images, maybe you would want to send this directory to someone else; including the thumbs.db in the folder could save them the hassle of generating a thumbnail for themselves. That's quite a particular use case.

Even if that was the case, why does Windows only provide the option to turn this caching on and off globally? Shouldn't the presence of a thumbs.db be on a per-folder basis, so you can keep your directories clean but include it for massive picture libraries you intend to send?

Note: I understand how to clean out / disable / VCS-ignore thumbs.db files - this question is purely about the logic behind their existence.

Answer 1

Wikipedia says that this functionality has since been moved to a central cache since Windows Vista, but thumbs.db remains for writeable network shares (I can't find the citation for that). This would seem to indicate that the current function of thumbs.db is indeed to spare others the time taken to generate a thumbnail cache. This is a changeable group policy.

This and OS X's .DS_Store files are so incredibly frustrating.