Because the people who enable to problem are part of the problem.
I'm a war you will cut off supply lines for the same reason, errode support. The commander doesn't say "people are dying on the front line and you're worried about stopping trucks transport bread? Get it together man."
I don't wanna beat the analogy into the ground so I'll let it go. But the point is, ones of them was going to be president. You will HAVE to deal with one of them. Refusing to vote is making the statement that you're ok with the worst outcome, because of you weren't ok with it you would have taken action to prevent that worst outcome. This is the general "you", not you specifically obviously, since you voted.
I feel like a lot of people thought "I don't like either, so I won't vote" ignoring that they will be impacted either way, so it's not a matter of who you like, it's a matter of effecting the outcome to get the best result possible. Even if that isn't perfect for you.