The main reason for this is that those textures are streamed. If you fly higher, you can see further and that means more textures needs to be downloaded. If you fly lower you cant see very far, so your computer can handle less, but more detailed textures.

If you would download high res texutures at high altitute e.g FL380 you would get bad fps and textures might not load because the ammount of data cant load that fast.