What if the assumptions they make raise the premium on your health insurance because someone sells your data? People (or, more likely, algorithms) making wrong assumptions, even if it is their own fault, can affect you negatively.
There's also the chance that those algorithms and assumptions are "correct" from a business standpoint (it would cost more to "fix" them than the monetary benefit of fixing them) even if they're not correct for consumers, meaning nobody that's actually in control of them has any motivation to fix them.