Phone manufacturers always buy the drivers from chip manufacturers, and under closed licenses. The source is rarely ever published, you can't download the kernel patches and do anything. Often you'll wait months or years to get the kernel the manufacturer used, and it will have been so old that none of the drivers will work on a recent linux kernel anymore at all.

The situation isn't exactly great. And torvalds believes it to be more important to talk nice to these companies than to enforce the GPL, so nothing's been moving in the past decade at all.