Dang, maybe you are really young. I lived through the massive theft of photography and art on flickr in the late 2000s. Rebekka Guðleifsdóttir was my friend there and her photos were being sold in London for more than 2,500 pounds without a penny going to her. She complained, tried to sue....no luck, she was utterly ripped off. All that while she struggled with being a single mother. It was tragic and every one of her contacts saw it go down in real time. We were horrified.
Maybe amateurs who don't plan to make a living from their work don't need to worry about it, but working professionals watermark their images for a good reason.