• Plastic is a good insulater
• plastic is a safer material
• plastic is more cost effective for necessary single-use objects like syringes and other medical equipment.
• plastic is more malleable, easier to mould and twist into useful shapes

Wider use of plastic pre-dates "wars for oil" and metal and timber are commodities that imperialists want to control just as much. They just have better control over those than they do oil right now.

Capitalism results in just as much human misery with regards wood, metal and glass production as it does plastic. There's no ethical consumption under capitalism because there's no ethical production under capitalism.