Why is the earth's core molten and not consuming everything around it?
It's not molten.
There are a lot of misconceptions evident in the actual question. First, the Earth's inner core is believed to be solid nickle and iron. Although it is very hot, it is under extreme pressure due to gravity. In the outer core the pressure is less so it can be liquid.
As for consuming everything around it, you need to know the law of the Conservation of Mass. The basis of it is that matter cannot be created or destroyed. So if you threw a rock into some lava, it would not be consumed...it would melt and just be part of the lava.
I apologize if I am misunderstanding what you are asking. If I am please either reword the question or leave a comment below and i or someone else will be able to help.