Like, you're marketing books. Surely you want a good book, not a writer so electrifyingly interesting they've attracted a cloud of followers just by being. I've never, ever bought a book just because I thought an author was an interesting person, but I've bought plenty of adequately marketed books by authors I've never heard of.
I think agents look at authors a bit like sports teams look at prospects. ie. they look at the ceiling and the floor of potential authors.
An amazing, new, innovative book from an unknown might have a really high ceiling, but amazing books get rejected by publishers all the time (just ask J.K. Rowling) so the floor is basically 0 copies sold, no revenue for the agent.
An author who has 250,000 followers on social media has an instant sales platform. Probably not everyone on their list will buy their book, but it's a safe bet many of them will. As long as the book is fairly good they'll probably get picked up by a publisher and at least sell 10,000 copies. With luck, timing, etc. even a mediocre book by a big social media presence can out-sell an amazing book by an unknown. It sucks, but it's simply the way things work. So this author has a pretty high floor, an agent can count on a return AND the ceiling isn't too bad either.
Sucks, but there it is.
On the flip side, if you're not already internet famous, there's not much point in trying to expand your platform (unless you have an amazing, outgoing personality and a unique spin to put on things) because to most agents the difference between 50 and 5,000 followers is inconsequential.
My first agent sent me a link to John Green's videos after I'd been with her for a few weeks with a note along the lines of, "Can you do something like this?" But that was when social media was just starting to explode and I think she didn't quite get it (a lot of agents around that time were going nuts over social media presence, be thankful that time has attenuated that trend).