Say for example, you have a folder full of movie files. The difference in computing required between just displaying a list of the files in there vs interrogating each file, generating a thumbnail on the fly and possibly sorting based on some metadata inside the file is probably more than an order of magnitude.
Not to mention all the extra stuff you have for security and stability when you are running so many applications at the same time.
You also have the classic trade-off of developer time (and therefor time to market) vs computer time.
Perhaps it's just me but I would happily trade in all that gimmickery for a usable file manager on OSX. I don't care for thumbnails.
I think I have tried every finder-addon and replacement under the sun. Nothing comes close to the frictionless and fast experience that (dare I say it) Windows98 Explorer gave me 14 years ago.
However there's a range of tasks where a GUI file manager is just the right tool for the job. I maintain OSX should have a proper file manager, regardless how much Apple wants the Filesystem metaphor to die.
I am merely pointing out the fact that I have yet to find a smooth experience on modern desktops like I found on previous systems years ago. No matter how much power we now have, it still feel damn slow, and maybe there is an excess of gimmicks vs actual, useful, clean functionality.