Why is the next generation of memory (DDR5, DDR6) always in graphics cards before it becomes directly placable in a PC?

Because it's GDDR memory, not DDR. They're two different standards that aren't interchangeable.

https://www.technipages.com/whats-the-difference-between-ddr-and-gddr-memory/

TL;DR it's like the difference between a GPU and a CPU. Similar acronyms, different function.

When it comes to motherboards, they have to develop the slot / physical form factor for the memory chips and get motherboard manufacturers to begin adopting it, whereas GPU manufacturers can just begin integrating the new memory on their boards when they feel the price/performance ratio is worth it.

(edit) - For clarification, GPU's don't have sticks of RAM the way a motherboard does. The memory is just soldered on the GPU along with the rest of the chips.

DDR5 (general compute memory) actually isn't the same as GDDR5 (graphics compute memory). I am no expert in this field, but you can read about the differences on wikipedia. This excerpt sums it up:

Their primary characteristics are higher clock frequencies for both the DRAM core and I/O interface, which provides greater memory bandwidth for GPUs.

Basically, GPUs need a ton of bandwidth but can withstand latency due to the predictable timings of the framerate.

I also assume that because the GPUs processing and memory are on the same board they have less friction in using newer standards. When dealing with CPUs the memory standards need to line up between different physical parts (CPU, motherboard, RAM).

The naming is not consistent so GDDR6 is more similar to DDR5 than GDDR5 is. So the difference is not that big but still a year or two.

It's easier to use new memory types when the chips are close by. Even phones adopted (LP)DDR5 before PCs did.