[__loam]

In addition to what everyone else has said, pixel art relies a lot of techniques like AA, dithering, outlines, specific ways of drawing curves, limited palettes, and implied detail that make it difficult to just convert from a vector piece. Two examples where conversions were used are Final Fantasy Tactics and the Advance War series, but as others have stated, you do have to go in and clean up the lines. There's also a nice feeling that's present in hand pixeled art that's hard to achieve from just conversion.