Earlier quoted context omitted.
Problem is that grabbing the last frame buffer would mean you're doing it post decompression. To distribute something you grabbed like that you'd need to recompress it and then have a generation degradation of lossy compression.
That's not how that works. It's one thing if you're converting from one compressed format to another, or recompressing an already compressed file. It's another to simply grab the pixels off of screen and compress those. You won't get as good of a shot because of the original decompression, but your description of why is inaccurate.
Everything I just said is rephrasing what is already in algesten's comment.