Well, "i444" stands for YUV 4:4:4, I think, while "rgb" stands for, well, RGB.
While the YUV 4:4:4 format is similar, in a way, to the RGB format in that it does
not subsample the chroma and takes the same number of bits per pixel (as "raw" data),
YUV (YCbCr) and
RGB still a two different ways of storing "color" information.
Also I think x264
does support encoding in the RGB (or BGR) format for quite a while now:
http://git.videolan.org/?p=x264.git;...edda65db0e6aea
(I think encoding in RGB might be less efficient than YCbCr, because the latter decorrelates the "chromaticity" and "brightness" information. Maybe also related to what the CABAC contexts are optimized/designed for?)