I had a GTX 970 that had 4 GB (actually 3.5 GB, and 512 megs of slightly slower VRAM for some odd reason). But I see what you mean. I only have 16 GB main RAM and my current card (GTX 1080 Ti) has 11 GB. But the answer to your question is, I'm told, the same reason why games nowadays are so yuge - Greedfall, 35 GB; Doom Eternal 45 GB; Metro Exodus 72 GB; Kingdom Come Deliverance 68 GB; Total Warhammer II 57 GB; Horizon Zero Dawn 72 GB - textures.
An image that's 320*200, 8 bit colour (i.e. VGA) is 64 kilobytes. That's not much. But make it true colour (32 bit colour) and it quadruples in size. Expand it to 1280*800 and its size hexadecuples as well, going from 64 KB to 4 MB. A true colour image in 4K is 31.64 MB.
Then add to that that in a 3D view you might be loading in and displaying tens or even hundreds of different textures on many different objects in view, and the space required explodes.
Unfortunately, data compression algorithms haven't improved in the same way as the space requirements for images.
That being said, I'm told that games like COD have uncompressed audio in their files. Fhat the wuck. FLAC exists, people. Use it.