ELI5 Why can’t we build above ground water reservoirs to stop the water levels rising?

I've always seen arguments about global warming and the icebergs melting equating to ice in a glass of water, as the water ice is already in the glass the water level doesn't rise when it melts so the icebergs melting shouldn't create rising water as it's already in the water.

Ignoring all that. Why can't we just build massive above ground water reservoirs, and pump the water into those? In places like the outback in Australia or in big desert areas that are essentially inhospitable. As the land mass is connected to the planet, surely the excess weight of the water won't cause a rise in water levels so it should help?

Can anyone explain it to me?