I think for many reasons, two of which I will offer here.
1) I think a lot of the time people don't realize they are missing information. As example, I don't think people know that you have to ask about the untreated group to understand information. So if someone says they have figured out that the best software engineers in their company are disagreeable, they don't know to ask if their unsuccessful software engineers are also disagreeable (maybe software engineers are just disagreeable!). Without knowing you need to ask for a control group, you might start looking for disagreeable people to hire, which would be a big mistake.
2) When the information someone already has confirms their priors, they will be biased to not search for new information (which might disconfirm their prior belief). So bias is a big part of why people don't search for information.
Why do you think people make decisions without considering what information they are missing?
I think for many reasons, two of which I will offer here.
1) I think a lot of the time people don't realize they are missing information. As example, I don't think people know that you have to ask about the untreated group to understand information. So if someone says they have figured out that the best software engineers in their company are disagreeable, they don't know to ask if their unsuccessful software engineers are also disagreeable (maybe software engineers are just disagreeable!). Without knowing you need to ask for a control group, you might start looking for disagreeable people to hire, which would be a big mistake.
2) When the information someone already has confirms their priors, they will be biased to not search for new information (which might disconfirm their prior belief). So bias is a big part of why people don't search for information.