Actually, it's a very good question. Rafflesia certainly acts more like a fungus than a plant. It doesn't photosynthesize or have traditional roots, instead getting it's nutrients and energy as a parasite burrowing into certain vines.

I'm sure there are many more reasons which could be explained by a botanist (which I am NOT), but I think one of reasons it's considered a plant is that it reproduces like other typical plants (i.e. with a pistil, stamen, pollen, etc.). It's DNA is a much more compelling case still. Examining its DNA (mitochondrial and nuclear) it is clear that it is a direct descendent from non-parasitic photosynthesizing plants of the order Malpighiales. Once rafflesia found its parasitic food source, apparently it's vestigial leaves, stems, and roots eventually simply evolved out of it.