Ok, so this can easily be explained, but you need to understand there are many reasons this is disabled by default.
First of all, Files app uses FOLDER CACHE (located in _files/cache/folders). This means, when a folder is viewed for the first time, a JSON data file is created for the folder. This allows the folder to load much faster when viewed
after the first time. When this cache file gets created, your images are not yet resized (because the folder has not been loaded, and resize requests have not yet been made). Therefore, the cached data file may not contain references to resized images, and it will not automatically update unless the folder changes.
You can try this: Delete files in _files/cache/folders/* and new cache will be created with reference to images that are already resized. Please understand, Files app cannot check if resized images already exist, unless we disable folder cache, but that would make everything much slower.
In the
Files app demo, I achieve this by first creating all images, and then force-recreating the folder cache AFTER all resized images are created. This is not practical, but is not too complicated becaiuse the Files app demo does not change much.
Also, why do you need to do this? It's not very productive. When Files app requests images through PHP, it will still output CACHED images, so it will still be fast. The benefit is that when routed through PHP, Files app can serve cached image (if already exists) and resize if it doesn't. If you want to always detect direct path to resized images, then it would not be possible to make it work with folder cache.
Also, keep in mind, it will not work with login enabled.