[Steve44]

It's to do with how fine these plastic particles are and the sheer volumes of waste which need filtering. They do remove a fair percentage but not all. [1]

Even in regular drains there will be a significant amount of contamination which would prevent the water from being used as-is; if you have a specific rainwater collection system that would work but is basically what our reservoirs are now.

[1] https://www.patagonia.com/blog//2017/02/an-update-on-microfi... "wastewater treatment plants filter a good amount of microfibers (65–92 percent) but still release a significant volume of waste into the environment"