There's big money in Medavantage plans. Agents get a nice commission for each new policy. They even get a commission for each renewal. That's because Medavantage plans have the highest profit margins in the industry.
I notice those TV ads gloss over that you're leaving Medicare and going with private insurance. They also don't mention that the insurance company will be much more inclined to deny your claims. And there's no mention that to revert back to original Medicare, you'll need to qualify. Good luck doing that.