Probably the same reason the rest of the toilet is -- it's strong, durable, scratch resistant, doesn't degrade with age (toilets can last for decades, my parent's toilet is original with the house, so is 40+ years old), and is easy to clean. It's heavy enough to not need any fastening mechanism to keep it on. And it perfectly matches the rest of the toilet, so you don't end up with a yellowed plastic lid on your bright white ceramic toilet.
> Probably the same reason the rest of the toilet is
Not all of the toilet is ceramic/porcelain. Seats are typically made of durable plastic and are designed to be replaced every decade or so. A plastic lid would probably last much longer.
