Polyethylene, usually added in the form of "microbeads", is a method of exfoliation. Biodegradable physical exfoliators are traditionally pretty expensive. Many, such as the infamous Saint Ives Apricot Scrub, are simply too rough for delicate facial skin. Plastic is an easy fix. It's cheap as fuck and can easily be made into even balls with no sharp edges, which is much better for your skin.
As nice as this may be for your complexion, environmental researchers have recently begun to examine the potential impacts of polyethylene microbeads on the ecosystem. The problem is that polyethylene beads are specifically designed to get through sewer systems, and, in that respect, they're designed a bit too well. They easily make it through waste treatment plants. Subsequently, they pass into natural water systems. Once they are released into an ecosystem, it can be absurdly difficult to remove them. What's more, polyethylene does not typically biodegrade, meaning that once it's in the water, it just hangs out as a pollutant. Consequentially, microtrash levels are at an all-time high. For example, in the Great Lakes, it all adds up to a record-breaking 450,000 bits per square kilometer.
A few companies like Unilever, Johnson&Johnson, and Procter&Gamble are all already in the process of phasing out microbeads. Researchers at the Virginia Institute of Marine Science are also in the process of developing microbeads made of polyhydroxyalkanoates, naturally-forming linear polyesters that serve as an effective form of biodegradable plastic. In the meantime, I will be avoiding cosmetic products that list polyethylene as an ingredient. This is a relatively small change given the potential environmental benefits.
Still, I would caution you against thinking that avoiding polyethylene-containing facial products is the end of the story. Most of the plastic microparticles floating in our oceans, rivers, and lakes are pieces of larger plastics. Discontinuing the use of polyethylene-containing products is a good start, but it only addresses a single piece of the puzzle. In order to help reduce environmental pollution by microplastics, we need to be more cognizant of our plastic use in general, and this can't stop with what we put on our faces!