To the best of my knowledge, Styrofoam is not a popular material for plastic recyclers. Recycling it at home eliminates transportation costs for this bulky and low-value material.
Many people don't own the property where they live, so maybe the incentive to build a castle for the owner over time is not there. Instead of filling our dumps with this stuff, we could be building cities out of it, if we were motivated enough.
I live on an island, and our dumps are filling up fast. Taking waste and finding constructive uses for it is the best way to gracefully live with all the trash we generate.
Styrofoam concrete probably has good thermal insulation, compared to rock concrete. It might be a useful construction material in both hot and cold climates.
Step 1: An early experiment
This is a porous block of Styrofoam and cement. By filing blocks of Styrofoam with special tool I made using lots of roofing tacks, I was able to make a supply of pea size particles.
Using a soupy cement and water mix, I got the particles wet with the minimum amount of the mix needed to keep them stuck together. By using the minimum amount of cement, the air space between the particles is not completely filled and the block is porous. The block is fairly light weight. It's strength is not what solid concrete would be, but sometimes the light mix is just fine for the job.
I don't know what kind of uses this combination might have, but its porosity is interesting. Perhaps, it could be a filter for air or water.
Step 2: Another experiment
This experiment uses a regular mix of sand, cement and water, and the porosity between the gravel-size particles is saturated with the mix. The resulting concrete is stronger than the earlier experiment, but also heavier.