I wouldn't call it massively underestimate -- if I recall the research of Meehl et al correctly, clinicians, on most types of cases considered independently, underperform simple arithmetical models of 2--5 variables by something on the order of 10 %. So not a huge effect, but also humans aren't as good as they think. (They do get lucky though! Sometimes some people get very lucky and accidentally get a long string of cases right.)