Its very simple 
plastic is handy and lighter and easy to manage 
also plastic made furniture are way cheaper than the wooden ones 
so its like that when you break a plastic chair you order a new one without thinking much but  when break a wooden chair you want to get it reapired but still order a plastic one in place of it 
people see their comfort before anything else 
everyone knows that plastic ruining our environment but no one cares (except a few stupid forest rangers and environmentalist )