How can Opinion Polls be seen as Useless

-2017: Predicted a Conservative Majority, which ignored the 'Youthquake' who then voted for Labour Candidates. This gave the Conservatives a Minority -2016 EU: Polls said the Remain Side would Win. Lol -Labour's Manifesto in 2017 had led to the Shift in Opinion of Public Spending [Not Opinion Polls shaping the Policy]