Because they are spineless cowards and various GOP operatives who have been doing this shit since Clinton and have become absolutely fuckin experts at it, came up to them in the locker room

“Hey so I pretty much everyone thinks Biden is a loser now,” they said.

“Really?” Said the media.

“Oh my God, do you even have to ask that?” asked the GOP derisively. “Did you see him at the debate?”

“I guess he did look pretty old…” said the media, thinking silently about when Trump said a wave of immigrants was coming in and killing our citizens at a level we’ve never seen.

“Media, he looked like a fucking ZOMBIE. He looked like an old, old man who didn’t know what year it was. I hope they find someone new after this. They’re pretty much going to have to. There’s no way they’re going to stick with that guy. He looks like someone’s great grandfather. He’s finished,” said the GOP forcefully. And then, after a carefully timed pause, they turned. “Wait, you don’t think he did GOOD, do you?”

And so on

And then the GOP operative got paid almost two hundred thousand dollars a year, and the media went out and wrote the story it was goddamned well supposed to write.