Because you stated 2 things as if they were fact, and I know at least one of them is wrong. It's okay not to know things, but definitively saying something is true when it's clearly not true proves that you state unfounded ideas as if they were facts, which can and should undermine people's trust in the validity of all definitive statements you make.
Some humans are more trustworthy than others. Even the difference between "I think fire is cold" and "Fire is cold" is immense. To err is to be human, but to err with complete confidence shows that you're a little too comfortable with erring.