Because over the years, an expert would have developed a way to look at the important factors and disregard the non-influential ones.
An expert may come off as presumptuous when arriving at a decision quickly. It is precisely because of the expertise developed over the years that the expert can isolate the important factors quickly.
To you, it may be a fast 10 minutes decision (and a fast 10 minutes decision cannot be good, can it?) But the expert took more than 10 years to develop that skill she used in the 10 minutes.
Do you know what is important?