It's actually very simple, and I am genuinely surprised that more people haven't thought of it already. The AI "optimists" led by Zuckerberg imagine the next 10 years, when all is sweetness and light, with a little foresight. The AI "pessimists" led by Musk foresee twenty or twenty five years ahead, when AGI is about to happen, and understand the truly existential threat to human survival that happens when AI/robots are VASTLY better at everything human, from emotionalism to creativity to raw computing power. Simple. And you better start paying attention now or AGI will creep up without you knowing, and obliterate all human life. It's that unbelievable -- all human life. I hope you are listening.

EDIT: And just so it's clear, lets say I'm wrong, and AGI doesn't happen for fifty years. If "climate change" were saying that in another fifty years all human life will be eliminated from this planet, and only AI/robots will exist after that, you would be alarmed and amazed. Everyone would be clamoring to stop it... Think of it like that if it helps you.