They can. Very easy to do but the problem is and will always be, if you have a good, great gaming machine by Apple, you also have a device that will run their Pro Apps and oddly, Apple assumes they will lose money as the Pros, 2~3%, would buy the cheaper alternative, and their right, we would but so woudnt the gamers (who make up more sales then video sales and music sales combined), as well as the millions of ProSumers who don't really have the cash for a server made desktop l, but do have the talent, nit to mention all the studios that had mac pros would also buy these non existent devices for their smaller rooms. In a nutshell, Apple would MAKE money not lose money due to lack of sales but they can't seem to figure that out yet.
Avid/Digidesing did!!!! They saw people were no longer buying their $$10,000 TDM (processing chips in a card, thus reducing CPU latency which you cannot have in music), and saw computers were getting faster and faster and more core on a single dye were happening, so they purchased M-Audio which relies on the cpu only. In music it's caled native recording vs TDM. In fact, for the mac pro, for those left with the express slot, now only the 17", you can purchase something similar that puts all the processing on the card that goes into the express slot. Google. UAD laptop express card and you'll see what I mean.
Anyway, Apple can do it but won't as I said earlier great grahics normally mean it can run pro apps and they don't really want that and rather have you buy the mac pro when in fact they would make so much more anyway.
Check it out. Let's pretend apple released a $1000 i7 core with a great graphics card, 1088 FSB, normal memory, headless and in the future you could swap the CPU out.
You would have thousands of gamers buy them.
You would have millions of musicians buy them and buy their own ram after market, similar to gamers.
You would have everyone that is tired if windows but has nice HDMI DVI diplays buy them.
You would have the more semi pro photographer buy them.
You would have most all semi pro video users buy these.
And ironically, all the pros apple were afaraid they would lose sale to, would buy these instead if the pc rendering farms as well as place them in their smaller studios, so it not a matter of how come, it more like theirnare misguided somehow. I mean their really only care about the iPhone now, one to one and pro care used to be one program. Now it's seperated and most of the training used to be pro apps, and is now iLife and $99 each. So why not go ahead and build them. There is a huge market out there. The only problem I would see is people would want their own video card at new egg or Frys pricing and apple would have to start supporting numerous cards but all the companies are gone and there's only nvidia and ati, so that wouldn't be that big of a deal and it would for sure, put a dent in the hacntosh.
There are millions of users waiting. Just take a page from what digidesign did.
They would have 20% of the marketshare within a year and to top it off, it would increase the sales of iMacs as business and enterprise started off with these mid range machines. 30% in less than 5 years. Is it really so hard to understand Apple?
Peace all.