Just because the brick is able to withstand throwing it on the rocks doesn't mean it is not going to deform under load.
Brick strength is measured in ability to withstand deformation under load and as far as I know there isn't any popular "plastic" that can do comparably.
For a single story house this probably doesn't make any difference, but for a larger structure the load could cause bricks to loose shape, creep and cause the structure to fail.
Also, plastics tend to get soft rapidly at temperatures.
I don't think plastic bricks exposed to sun (Kenya?) would fare for long.
Another issue is that plastics tend to deteriorate in UV. That could be fixed with a good paint but it also costs.
(Edit: for the pavement this is horrible idea as the plastic is going to be broken down and dispersed in environment in large amounts)
Brick strength is measured in ability to withstand deformation under load and as far as I know there isn't any popular "plastic" that can do comparably.
For a single story house this probably doesn't make any difference, but for a larger structure the load could cause bricks to loose shape, creep and cause the structure to fail.
Also, plastics tend to get soft rapidly at temperatures. I don't think plastic bricks exposed to sun (Kenya?) would fare for long.
Another issue is that plastics tend to deteriorate in UV. That could be fixed with a good paint but it also costs.
(Edit: for the pavement this is horrible idea as the plastic is going to be broken down and dispersed in environment in large amounts)