I don't know if people genuinely think that putting multiple -EV picks into a parlay them +EV.
E.g. there's a fav at -250 and someone might say "no value at these odds but good for a parlay"
It's not good for a parlay, it's a -EV leg of a parlay, therefore it shouldn't be there at all.
I believe that if you asked the average bettor if he'd bet on a coin toss at -110, he'd probably say no. But offer him a 5 coin toss multiple at +2400 and suddenly he thinks it's not a terrible bet.