Apple’s new Mac Pro contains plenty of space to add up to six PCIe cards. But in a bit of irony, the desktop PC doesn’t support third-party video cards—which often sit in a PCIe slot.

Apple executives addressed the missing graphics card support in a talk this past weekend with blogger John Gruber, who runs Daring Fireball.

“Fundamentally, we built our architecture around this shared memory model, and that optimization,” John Ternus, Apple SVP for hardware engineering, said... during the event. “And so, it’s not entirely clear to me how you bring in another GPU, and do so in a way that’s optimized for our systems.”

To power graphics, the Mac Pro has been designed with its own built-in GPU through Apple’s M2 Ultra chip, which can span 60 GPU cores or 72 GPUs cores. The same chip can access the Mac Pro’s shared RAM memory, which can be configured at 64GB, 128GB, or 192GB. As a result, Apple says the Mac Pro’s GPU can access far more video memory than other workstation graphics cards, which only have 32GB or 48GB of memory, for example.

“With that 192GB, yes, it’s less than 1.5TB [of RAM found in the 2019 Mac Pro], but it’s way more memory for the GPU than any GPU has ever had before,” Ternus said.

Still, Apple’s decision to nix the third-party video card support may hurt the product’s appeal, especially since graphics cards from Nvidia have become vital to training AI algorithms for programs such as ChatGPT.

In response, Apple’s SVP for marketing Greg Joswiak said, “Look, Nvidia is doing a good job with that. There’s no doubt about it. We wanted to focus on the most important things to our customers and we do those things well.”

“The things that we can do with our unified architecture and with our M2 Ultra are things that nobody else can do,” he added.

Although the Mac Pro doesn’t support external video cards, the PCIe Gen 4 slots can be used to attach networking or storage cards. This can make the Mac Pro better for high-volume audio and video-editing tasks over the smaller and more affordable Mac Studio mini PC, Ternus said.

The new Mac Pro goes on sale June 13, starting at $6,999. But at maxed out specs, it can reach over $12,000.