Speaking from experience, it is much, much, much more feasible to reach the burning point of wood or the melting point of plastic in a datacenter than the melting point of the metals used in server construction.
One of the jobs of the case is to contain a fire if it starts. A plastic case wouldn't do this, and a wood case would actively be hazardous.
Do you have a story? I've never experience more then 120°C in electronic equipment as it's usually the safety limit where it would shut itself off. Although in theory the silicon would still be fine at 300° where wood starts to burn.
1) Several appliance machines were in a cabinet with water-cooling doors attached. The doors restrict the airflow from the front of the cabinet to the back to the point where there isn't sufficient airflow to cart away the heat from the CPUs. The engineer responsible for those particular systems played show-and-tell with the melted plastic pins that formerly held the motherboard in-place for a month after the machines crashed.
2) An 8-year old rackmount machine had a power supply fail spectacularly and light the entire server on fire. Fire suppression was triggered in response. Several other adjacent machines were damaged but the fire stayed relatively contained to the one cabinet.
One of the jobs of the case is to contain a fire if it starts. A plastic case wouldn't do this, and a wood case would actively be hazardous.