Plastic bags cannot be sewn. Sewing only works with fabric, which is why it can't be done.
It's not that sewing machines aren't powerful enough--it's just that the threads would snap in a nanosecond because there are no stitches strong enough to hold up against them. But you know how thick plastic bags are! Sewing wouldn't have any chance at all of penetrating through the material, so your thread would never get anywhere near the fibers to sew them up in the first place