Decompressing something to use during rendering of the same frame probably also isn't worth the latency penalty. Compression is fine for streaming data, but not so much for low-latency seeking around within data.
Yes, but they have special formats (ETC, PVRTC, ASTC etc.) so GPU can use them directly, and the compression is lossy. e.g. PNG would not work in this situation because GPU cannot decode PNG textures fast enough.