It's on the aggregate of predictions that you can see the accuracy. If my weather app tells me hundred times there is 10% change of rain but it rains 50% of the time after 100 days, it's completely useless.
Same for FiveThirtyEight. They have been massively wrong on multiple counts.
Sure, but generally because the aggregated polls were systematically off. You may be thinking of other examples, but the 2016 US presidential election really wasn't that bad of a look for 538, they had Trump at reasonable odds going into election night. It's not like he was at 5%, 538 pegged him at 1/3 odds as polls opened if I remember correctly.
