Why are most enamelled cast iron dutch ovens only rated up to 400-450°F?
They sometimes have plastic (cast phenolic resin) knobs on them that are only rated for just over 200°C (400°F), or other decorative trim that is not suitable for high heat due to deformation etc.

They do tend to start crazing (surface cracking) at over 200°C too

The enamel (a special soft glass powder) is fused on at over 800°C (1470°F)

You cannot melt glass enamel on a domestic stove. It would have to get to over 1100°C (2000°F) to melt!