Drawing only 2W and producing no noise while streaming a 1080P movie is a pretty good selling point of these devices vs. a full computer. Apple actually tried your way, since the original Apple TV was just a variant of a Mac Mini. It sold better when they went to mobile device specs and $100.
I have a different vision I’m moving towards: All the media on a server, sitting where nobody sees it. Stream from there to all the devices, including the computers, iPad and even the phones. I’m thinking 4x2 TB in a RAID5, giving me 6 TB storage. It needs a basic case and yard sale grade video card and LCD monitor, but a good mobo with SATA RAID HDD controller and a pretty powerful CPU to re-encode on the fly for low-bandwidth mobile clients.
Sorry but a proprietary box that does one thing and doesn't allow me to store media the way I wish is no selling point for me. I use my settop computer for everything and it does it all very well and it gives me options that Apple won't allow. I can create and edit video, photos and music. I can play games including the latest MMORPGs. I can watch Television via a inexpensive card AND record the programs In 1080p with Dolby Digital sound and store them on my hard drive. I can even watch blu-ray and if I want rip the disc to my hard drive so I don't even need to mess with loading a disc. (Or i can use the digital copy that is provided with many discs now but they still feel the need to put DRM on the damn things to make inconvenient to back up the data etc.)
These single purpose boxes are just an attempt by "Big Media" to hang on to their failing business model. (Reselling the same media over and over in new unneeded formats.) In a few years they will have a new and improved format which will require a new box and all your old media won't work on it. Not gonna play their game.