Every time I've seen him jack something up, he always puts a jack stand under the car. That's not to say he's done it every time, just to say that of the hundreds of videos I've watched of him doing things, he's never not used a jack stand when he's jacked something up and crawled under it.
Then you haven’t paid attention to his videos. There’s at least three times that he’s done it on camera. If I wasn’t at work, I’d go dig up the videos from my “Scotty is an idiot” playlist on YouTube.
Also worth noting that:
1. The guy has had several videos where he was visibly drunk.
2. He has stated as fact many things that have been easily refuted.
3. His mechanic shop in Houston was so successful that he went out of business (not sold out, went out of business) due to lawsuits and lack of customers and had to move in with his kids in Tennessee.