For Oreo and Eazy, Clooney raises gazillions of dollars for Dems, also has known Biden for ages. He recently said that Joe's recent decline is quite substantial, compared to a couple of years ago, so he has a perfectly valid opinion (as Vintage pointed out).
Sadly, at regular intervals, the USA political scene gives the public a choice of 2 lemons, as Pres, and they end up picking a lemon (albeit an orangey one in 2016). Given the popularity of a number of screen stars, sports stars, etc, why wouldn't they have a great chance if running for President?
Tom Hanks, George Clooney, Costner, Denzel Washington, Tom Brady, Peyton Manning, (pick your own favs with some gravitas) -- all look good and sound good, it wouldn't matter much which party they stood for, would it? So much of modern-day politics relates to what people look like, and can they do an effective sound bite?
Imagine if, say, Steve McQueen had not got sick, but he'd gone into politics? Or Eastwood when he was maybe into his 60s? See some 'normal' political candidate trying to debate, or make competing commercials, against the looks those guys could do to camera?
And just because they're not experienced politicos, doesn't matter much. They are less gaga than Biden, and not repulsive pieces of filth like Chump. Reagan got into politics, did fine with his assorted advisors (as they all tend to do anyway).