Completely different medium. Covering a song doesn't take nearly asm uch to remake a film. And most of the time, they're done for different reasons. Most covers are done as a tribute to inspirations, some done to convey their emotions in someone else's words, and some just to redo something popular. That last reason is the one behind most cinematic remakes. I hate it when people remake already great films just to bring a breath of fresh air into it without accomplishing anything. Leone managed to experiment his style with a remake of Yojimbo, but it worked. However if say Scarface was remade tomorrow, it would be to take advantage of the original's popularity like most remakes these days are.

But like I said, I can pick up my guitar right now and play a Nirvana, Alice in Chains song, whatever. It's entirely different from me directing a remake of a film. It may not sound different, but it's much more convenient and easier to do.