When guns are made of plastic, OK, glass filled polymer, it needs more volume to stuff in more materials and a trapezoidal geometry to create a "honey comb" to get to the strength needed.
hence, all the plastic guns are big, like the G36, ARX160 and the camel, and they are actually no lighter than SCAR, M4......etc. With the amount of plastics and metal inserts, they all have super skinny barrels to get down to the weight to be competitive against aluminum guns. At the end, plastic guns are no lighter than aluminum guns. Just like Pmags are heavier than aluminum mags.
So why is it plastic guns so popular? They are cheaper to make in volume in countries (especially in places with highly skilled labour cost) that do not buy 100K M4 a year, when you make that many guns the production scale is up another notch and the cost of doing aluminum comes down below plastic molding.
But plastic gun is not easy to make right either. Malaysia tried to make AUG and they screwed up royally. They ended up buying Colt's M4 license and back to M16.
I think the allure of plastic guns mainly occurs in the 2nd world countries these days, and the idea of licensed manufacturing and technology transfer. The supply of higher end aluminum stock to these places may be restricted too, so they tend to like plastic.