What LOD does is it uses multiples of the same textures in different sizes so that it doesn't display the larger ones if it doesn't need to. That takes up space due to duplicates, but 4K resolution textures take up 4 times the space that 2K resolution textures do. I'm sure compression reduces some of that, but in terms of size, they are 4 times larger. So if your system can't handle 4K textures, then why use them at all? There's a lot of stuff that you'll never look at close enough that a 4K texture will ever serve a purpose. For a 1080p screen, you'd have to be close enough to the object that you're only seeing a fraction of the texture at once, and they can use other tricks to make close-up textures look better without using bigger ones.

If you have a top-of-the-line PC, it makes sense to install those huge textures, but if you're running an old GPU with 2GB of memory, what use do you have for them? You may as well not install them at all.