I’m not a scientist but I think water that deep responses differently than solids like rocks. The carbon dioxide that great deeps turn into a kind of dry ice too.
Could also rocks are closer to the Earth’s mantle.
But don’t take my wode on it, I only got a high school science education.
“The carbon dioxide that great deeps turn into a kind of dry ice too”
only if it is below the critical temperature for CO2, which is 31.1 C.