As others have said, Marketing is most likely the reason.
What if they started shipping the boards with an onboard ASIC chip as a means to help distribute the network? Team up with Bitmain or someone and get ASICs cheap, toss one on the board with a basic heatsink. It would add ~5 watts to the power draw of the system or so, make the chip configurable in the BIOS so the user could turn it off totally or point it to their own pool or solo mine. Then it could be called th BTC ASIC motherboard.
People used to have that idea long ago. I read that some people placed usb miners inside their pc cases, so that they would start mining every time a pc was turned on. What if they started shipping the boards with an onboard ASIC chip as a means to help distribute the network? Team up with Bitmain or someone and get ASICs cheap, toss one on the board with a basic heatsink. It would add ~5 watts to the power draw of the system or so, make the chip configurable in the BIOS so the user could turn it off totally or point it to their own pool or solo mine. Then it could be called th BTC ASIC motherboard.
Putting small chips in every MOBO would be a great idea to support the network, but it would also make the mobo less power efficient and would require additional software to connect the chips to the network.
The user wouldn't earn anything from it, so you'd have to buy this mobo to support the network for free.
Don't know about you, but I would only buy it if it was really cheap. If I were to pay extra just to support the network I wouldn't do it. I'm not that charitable. On the other hand if my normal mobo would get the bitcoin chip added for free, sure, why not?
I was one of the ones who had this thought years ago and while I do agree with you (same arguments were made then) to some degree I just dont see it as a big hurdle. With the latest ASIC chips, the manufacture should be able to lower the clock and make the chips run very efficiently, maybe around 2-3 watts. Youre case fans total more watts than that and if it were an issue, they could include a BIOS setting to disable it all together. Id personally run it as some sort of BIOS application or hardware level rather then being dependent on the system OS and software. The added cost of the ASIC 'shouldnt' be much more than a few bucks. How many boards have you ever seen that use Firewire? Lots. How many times have you ever used Firewire? Personally, Never. And how many times did I look at the price between two boards and see a difference between a board that included firewire and one that didnt... never. I could be totally wrong but I dont see it as an issue.
What I do see as an issue is the idea of who you would be mining for; and this is a major issue that I dont have a real answer for. Would they mine to a pool and get a micropayment each week/month to their own address or solo mine and have to monitor a 'lottery ticket' for the possibility of a 'win'? Would they just mine to the manufacturers wallet and pay the electric cost unknowingly?
I think one solution could be that it solo mines to a solo pool setup by the manufacture but have the option to change to a user defined pool. Then have the user register their address to the manufacture (or a third party service) with an email address so that if a block is mined they will be notified. This way, a user has full control - they can change the pool, they can change the address, monitor themselves or disable all together.
Hm. I dont know. I dont have an answer but it would be really great to see the network distributed like this. Just some thoughts.
Sorry for taking this thread off topic.
when basically there are more practical machines named ASIC which only has one purpose that is to mine bitcoin, while you could mine with GPU but you are in more profit if you point that power towards altcoins. 
