"Why do liberals keep saying vote harder"
Because in order to do anything more complex you need to do the easy stuff. Far too many progressive-leaning Americans don't bother to vote. You vote in every election, from county dogcatcher on up to the new office of "Presidential God-King"? Congratulations! You, hypothetical reader, are, at least on the vague center-right to far-left blob of American politics, a wild exception. The most common political attitude in this left-ish blob is "voting is a pain, do I really have to? I'm not hyped about the candidate. I don't matter."
Yes! You matter! In every single election! From now until forever!
And yes, this is true even if you're in a safe blue state like California or New York, and are voting for President. Massive vote margins there let campaigns spend their money in the states that actually decide the electoral college.
Americans aren't unique here, I'm picking on them because they've got a wonderful election going right now. Canadians are, if anything, even worse. Its depressing.
This is also why "winning every election is not a strategy" is a nonsense statement, born out of the American system's tendency to gridlock. In a real political system, losing an election is catastrophic. The new party in power will rapidly move to un-do your priorities (even the nominally popular ones) and enact their own. There is no such thing as a safe election to throw on principle, to "send a message" about how you're unhappy with leadership. The only message you'll send is that you won't do anything, so they'll turn their attention to finding people who will.