Because they could vote for who ever is promising to allow chips every day in the school canteen (or what ever junk food is in vogue).
Or because a playground bully could be pressuring people to vote for vote for legalising alcohol at 16 without any sensible experience of drinking at all.
Or because they have limited experience of life outside a sheltered school environment and vote for policies that cost lot of money without the experience of knowing the consequences.
Or worst of all they are 'brainwashed' by social media because they have young vulnerable minds. (because no one would ever do that would they?)
All of which apply to adult voters.
They can and do vote for the someone standing in front of a big red bus dangling a carrot in front of them.
They can be pressured by employers to vote for reducing employment legislation red tape. ("What's good for the business is good for you/what's bad for the business is bad for you.")
Adults with life experience vote for batshit policies all the time. Literally every election.
And the last one? Sigh. Does that even need a response? You really shold have come to the conclusion on your own that millions of adults are unduly swayed by both social and print media. "I'm not voting for Ed Milliband cos he can't eat a bacon butty innit?!" "I'm not voting for Corbyn cos he didn't bow at the cenotaph!!!" "I'm voting for Brexit to get rid of the Muslims!!!"