I think most people use tablets to watch TV in bed. Now that iPhones and Androids are shipping with larger screens and higher resolutions, there isn't much need for a second device.
Similarly Google started out allowing bi-directional file transfers, but then later rolled that back to only allow reading, not writing, with regards to removable storage.
This leaves Windows. But there there fusing of Windows Phone's UWP and NT's win32 is awkward at best. Resulting in the best use of the "tablet" to prop it on a surface and attach a mouse. This because UWP file management is highly restricted, leaving us to use touch-hostile win32 programs for that task.
Yeah, some OEMs include a minimal file manager with their devices. But they are a pale comparison to what third parties can offer.