It’s capitalism. The market has determined that plastic in the ocean is the most efficient way to produce goods and services.
Update: a small grammar fix.
As long as we're using natural gas, there be a surplus of cheap ethane and we'll indirectly subsidize plastic, since making plastic out of the ethane puts that waste product to use (by creating plastic waste instead).
The problem is not Europe (outside of gypsy camps and illegal immigrant camps). It is mostly some parts of Asia (and it would likely also be some parts of Africa if they could afford as much plastic).