I am a plastics guy of 35 years but even still I can only apply my experiences and use them to logically speculate as to the answer.
Short version is " Thermal Conductivity "
Long version is ... While we can rarely confirm why something does not exist the temperatures are the likely cause here. The temperatures we see on the outsides of aluminum and iron manifolds or risers are much different than those on the inside before the water becomes effective at heat transfer and are too high for plastics to endure. I say this well knowing that aluminum risers exist and potentially could melt 100 to 200 degrees below what are high potential EGT's but in aluminum's case it's thermal conductivity properties help to pull those temps down enough to protect it in this situation and keep the casting from heat soaking to the point of melting and allow it to work. Same reason there are no aluminum exhaust manifolds on cars. The thermal conductivity of plastic is just no where close to either iron or aluminum and thus not suitable that close to the combustion temperatures as it would rapidly erode internally from the extreme temperatures.
Another consideration is aluminum being $7.00 ish per pound and iron even less is a relative bargain as compared to some highly engineered plastics at more than $20.00 per. pound these days. If it was to be attempted it would require high end expensive grade resins and may even negate all the advantages other than weight.
I am more curious why we have not seen internal coatings that prevent corrosion and make these components last as if they were made from stainless steel.
