Something I just found out about

[auroraglacialis]

Raiden:
a) it is waste. A mixture of all kinds of plastic and other materials. To separate this would be as tough as to separate household waste that was not separated for recycling. The only thing that one can make out of such a mixture without investing a lot of manual labor for separation is this black building material - or to burn it for energy, just like any other mixed household plastic waste.
b) to collect this is a huge challenge. The plastic in the ocean is widely spread, you'd need a fleet of ships to collect it all and in the collection probably you would kill many animals as well. These ships cost money and need fuel. If it washes ashore someplace, of course you can collect it, and this is done in many places to keep the beaches clean for tourism. In these cases that trash goes the same way as any other household waste - either to be separated and recycled, burned or in a landfill.