Because Windows doesn't provide a 16 bit print pipeline as of yet (not Epson's fault BTW) It's a Windows thing...
Just to avoid confusion, a Windows OS dependent driver is not strictly required to still produce 16-bit printer output, but it is the common limiter in most other cases.
For example, besides dedicated RIPs, also Canon offers an Export module for their printers that allows 16-bit print data to be printed. I'm not sure if Epson offers such solutions.
Print Plug-in for Digital Photo Professional
This export module allow users to directly process and print RAW image files from DPP. This plug-in bypasses the operating system's dependent print driver, allowing processing of 16-bit RGB images directly, which dramatically increases gradations and as a result, overall image quality.
Canon also provides an Export plugin for Photoshop that utilizes the 16-bit data, but ultimately still uses the Windows OS driver path to send its print data.
Print Plug-in for Adobe Photoshop
This export module allows 16-bit image processing, preserving the expanded color spectrum and high gradation of the original file. The Print Plug-in reduces the possibility of incorrect settings, by providing an integrated interface for print settings in Photoshop and the printer driver, enabling efficient production. In addition, the plug-in supports an Adobe Color Conversion Module (CMM) and proof printing is now possible by simulating the colors produced by printing presses.
Similar solutions could be created by other printer brands. So to only blame the OS is not the full story.
How much of a difference the 16-bit pipeline makes to output is another discussion, and is also dependent on image content.
Cheers,
Bart