The polls said it because polls are utterly obsolete and useless the way they are currently performed. They're a relic of the past. These days younger people screen their calls and don't tend to trust pollsters either and give wrong answers. The polling is no longer accurate over the population because the population is more and more the younger generation (not to mention next to none have landlines which is how many polls are still conducted).
The reason there was no red wave is because the youngest voters went out and voted against the republican party, and the polls didn't grasp that because they, again, are a relic of the past.