The Steam Machine is slightly bigger than a portable Blu-ray / DVD player. (I just checked this with my own players)
I can’t wait for someone to make the equivalent of the Gamecube’s GBA Player for discs. Because of its size, It could even be a plastic shell where you put an existing Hitachi/LG portable drive without having to disassemble anything
The Steam Machine is expected to be hooked up to your TV in 99% of cases. Why not add the capability to play Blu-Ray movies at that point?
Also, there’s an official dock for the Steam Deck, which is also intended to be hooked into the TV most of the time. It’s reasonable to want an attachment for that as well if you only have a Deck