I've used several worm drive saws over the years (but never owned one), and now that you mention it, all of them had blades on the left. Some of the other good responses may hit the reason. Several of the "regular" circular saw mfgrs offer in left hand versions.
DL, Makita has a 5 1/2" trim saw that may be exactly what you're looking for. It has a left hand blade, ball bearing drive, plenty of power, smooth as silk, comes with a nice carbide blade, very light and handy to use. They're not cheap, I got mine on clearance at Lowes and it was still ~$115, I think it was regularly ~$160. It's a pleasure to use and perfect for cutting sheet goods and 1 by's. Max depth of cut is about 1 1/4", so it's not suitable for framing. My regular 7 1/2" saw gave up the ghost and I replaced it with a Makita 7 1/2" and it's also very smooth and a pleasure to use.