That’s because they have NO sense of humor.
I thought about it when I saw your post, and again, asked myself “Why is that? Why are Leftist memes unfunny and ineffective?” and I realized:
They don’t have the truth behind them. I believe for a meme to be effective, it must have a basis in truth, something the Leftists cannot often leverage.
Sure, memes not based in truth can be funny, but it is harder for them to be effective unless there is truth behind them.