Well, the point is many of these streamers do in fact use a variant of a commonly available open source operating system (usually a Linux variant) that will run on just about any standard PC or compact Intel based motherboard in a custom chassis.
Rather than reinvent the entire wheel (operating system & Intel architecture), many streamer companies will take a Linux variant and cut out bloat that is not pertinent to a streamer and limit user access to functions that the manufacturer decides he wants in his 'appliance'. He also shields the 'appliance' user from other complexities that lie under the hood.
Many products are made this way, not just streamers.
The benefit is that the user sees it as a complete turnkey product while the manufacturer's time to market is quicker. They can manufacture a product that is repeatable and also more easily maintained. By limiting what the user has access to, they also have less to support when he calls for help.
But it's still fundamentally a computer under the hood.
