Because when you're talking to them directly you ask them about it. This is what I mean by all forms of feedback.

I'm getting the impression that people are just ignoring all advice about communicating with customers in their startups and just throwing stuff out there to see what sticks. Besides being wasteful in doing stuff no one wants anyway, what if that bad feature crippled your product and your paying customers have permanently switched to a competitor the instant your change frustrated them? And now you're bankrupt and can't afford analytics. Relying on analytics as a crutch to catch these things was the mistake in the first place.

Fun story I actually had forgotten about till now: I briefly worked at a tiny startup out of my school in the ending days of the internet bubble, trying to sell a "data mining" software product. Way too soon before it was cool, sadly. It was really hard to make the case that people needed to pay us $100k for the benefits we could get from their data. And companies certainly weren't going to go for a pitch like "if you launch a broken product we will catch that fact". They spend a lot of money to be sure that doesn't happen already. We even had one major customer figure out that they could just have an engineer perform a simple counter over incoming communications that would catch all they needed to know, and hence they didn't need our product anymore. That was kind of the end for us, in fact.