I would say the opposite is the case. And your cynicism reveals you don't know a lot of about the elite scene.
Many elite athletes I've spoken with see their sponsorship as a job, and not "free money." The problem is that they don't clearly know what their job entails. They, perhaps naively, see it just as you describe it: the job is to train hard, race well and hopefully place and represent your brand at events.
The truth is that many brands are now looking for that large social media following that you think is valueless. And many athletes don't realize this, and instead keep their head down and just, well, run.
Brands should communicate to their athletes what they are looking for, and athletes need to realize that they are a marketing tool, and nothing more. How they are being used as marketing is up to the brand's strategists, and they need to find athletes that work for them, and also communicate that plan.
From a Canadian perspective, Sheila Reid, Nate Brannen and Evan Dunfee are great examples of the brand-athlete relationship breaking down.
For Sheila, Nike clearly wants dominant yet relatable North American athletes for that market. They need to kick ass, but also be seen as personalities or heroes. That's how Nike has always built their brand. Sheila unfortunately didn't perform well enough for Nike in the last Olympic cycle. But she does have a great social media voice and is a marketable personality. That might be enough for a brand like Oiselle, but it doesn't fly for Nike (see what I did there!).
Brannen is a workhorse and a total pro. But he has zero social media personality. Saucony has recently cleaned house. They clearly want more traction online. Results aren't enough. Also, they don't think Nate has that much mileage left in him, is my guess. They're making a business decision. It's awful for Brannen, but he's involved in a marketing exercise, unfortunately.
Dunfee is great (albeit grating at times) on social media. He's punching way above his sport's weight in terms of reach and intrigue. But he's still a race walker. Asics got max value out Dunfee at Rio and can't do much better than that with him because... he's a race walker. They see no value in RW outside of an Olympic year, so all the snarky tweets in the world aren't worth giving away a pair of Gel-Hyper Speeds to a guy in a sport that no one actually cares about. If he'd won that medal, maybe things would be different because he'd enter the in-club. Again, this is all marketing, so fourth place is perhaps the least interesting athlete in an entire competition, from that standpoint.