As a PR agency that works with a ton of startups, I also see the same thing that Peter sees.
The first problem is, most startups are driven by engineers. Thus, their preconceptions of marketing are usually already skewed right off the bat.
For those who do understand the value of marketing, how to execute and test the various channels is the next hurdle.
This has become more complicated by the proliferation of marketing channels, creating confusion due to the plethora of choices.
Here's a real-world example: we were working with a consumer electronics company who was launching in the US. Within 6 months we got them covered and reviewed in Engadget, Forbes, USA Today, Techrepublic and soon to be CNET, among many other places.
However they felt that their marketing dollars would be better spent creating high quality photos and videos that they then post on social media.
The problem, as I've pointed out to them, is that organic reach on social media is near zero these days. Their internal Facebook stats bear that out. Their preference is to focus on creating content which no one is really seeing at the moment.
The point of this isn't to say "PR good, social media bad". It's back to Peter's point about qualified marketers versus people who dabble with Instagram and think that qualifies them to drive an entire company's marketing strategy.