Question

why are cooking vessels made of metals but that handles made of plastic?

Answer 1

The handles of cooking utensils are made up of plastic or wood because plastics and wood are bad conductor of heat. As they are bad conductors of heat therefore they won't be able to pass the heat of the flame to the person's hand and will prevent the person from burns or any other injuries.

Answer 2

Cooking vessels are made up of metals  because metals are good conductors of heat. So they provide heat to the utensil and helps in cooking.
Handles of the utensil are made up of ebonite (plastic) because they are bad conductors of heat. This helps us touch the handles. If the handles are made up of metals, it may cause burns.

So, handles are made up of ebonite (plastic). 

Hope this helps you