Apple does allow upgradeable video cards. Mac Pros use full size PCI cards.
Oh, so I have to pay $2500 for the OPPORTUNITY to upgrade? Yeah, no.
Most macs sold are iMacs and Macbooks, which have crap video cards. I'll repeat this again for those who don't know: Apple DOWNGRADED the video cards in the glass + aluminum iMac Vs the previous model. The ATI 2600Pro isn't as good as the NVidia 7600GT (at most games), even though it is newer and is DX10 compatible. The 2400 (in the bottom iMac today) isn't even worth mentioning.
iMacs Use the MXM style cards with Desktop class GPU's instead of the Laptop class GPUs normally normally found on these cards.
I doubt this is the case with the new iMac. It seems to be a modified laptop GPU. Not that it matters because even the 'desktop class' 2400/2600 is crap.
The only thing stopping you dropping a different card into the Mac is that almost all the cards on the market only support the BIOS firmware, where Apple uses the newer EFI on Intel based Mac's and Openfirmware on the older PowerPC.
Uh the cards can have new firmwares written for them and it'll work fine.
The Firmware + Driver combined bridge the barrier between the OS and the video card. Oh, and it's all software. You could just as well have almost no firmware and make up for it with a driver.
In fact, there's a driver out there for many 'BIOS-compatible' PC NVidia cards that lets them run in OSX! Look it up, it's called 'titan'. It's written by some dude in his mom's basement.
The point is, if Apple had ANY desire to put better cards in, all they would have to do is write or pay someone to write new SOFTWARE. There IS NO HARDWARE BARRIER between Mac and PC video cards.
I'd say 90% of Apple computers sold today are NOT Mac Pros, which means these 3rd party graphics card vendors have NO REASON to write a Mac Firmware & Driver unless they are BUNDLED with the computer.
Apple has shown us, with the crap video cards it chooses, that they have no interest in paying a higher per-unit cost to include a better video card.
To be fair, their sales have shown us that they really don't have any reason to care
Find a card that supports designed to work with EFI and you have a new video card for your Mac. Which is pretty hard except for the cards Apple already offers.
Yes, for the reason I mentioned above: Most Macs are not upgradeable, so the upgrade market is non-existent.
Moreover, the driver is equally important to the firmware. In fact, the firmware is really just part of the driver that happens to be on the card itself.
This whole EFI vs BIOS is really built up. The real reason we have no cards is because of software.