A lot of reasons.
First and foremost, players are often bad at identifying what they actually have a problem with. A player might tell you "the dodge mechanic doesn't work" when the actual problem they're having is that the boss's attack isn't telegraphed well enough to give them the advance warning they need to use it. They might tell you "there isn't enough content" when the actual problem is that you set the leveling curve too shallow and they're outleveling 90% of the content in the game before they ever see it. They might tell you an enemy "has too much health" when all they really want is a better visual indicator for how close the boss is to dying, or that a gun "sucks" when mechanically it's fine and all it needs is more satisfying visual and audible feedback.
They're even worse at identifying how to fix problems, which is an issue considering most players want to do so without even having stopped to identify the problem properly first. To take the example above, a lot of players will skip the entire "the dodge mechanic doesn't work" part and go straight to "you should replace the dodge with a block"; basically they'll give you (potentially bad) advice about how to fix something that isn't even the actual problem.
They're also prone to being forgetful/having a recency bias/etc. If you actually run biometrics moment-to-moment--eye-tracking linked to heat maps on menus, heart rate during specific moments, etc.--you'll find that they often neglect to mention the important points and instead project the feelings from those onto the broader experience or later parts of it. A player who gets really frustrated with a bad menu might neglect to mention that entirely, and instead tell you they didn't enjoy the entire sequence where they were struggling with the UI, even though it had nothing to do with the rest of that sequence.
Think of it like going to the doctor: tell the doctor your symptoms in the simplest terms, don't try and self-diagnose and offer possible prescriptions.
