Because microplastics move up the food chain they eventually get to fish humans eat. This matters because plastics contain a variety of harmful chemicals such as phthalates, BPA, and petroleum. These chemicals are horrible for your body and result in things like cell death, hormonal issues, hormone-related cancers, neurodevelopment disorders like ADHD, autism, and infertility.
All types of sea animals find plastics in their ecosystems. These animals often confuse these plastics as food. This problem is so big that scientist estimate that by 2050 99% of seabirds will have human waste in there stomachs.
Things like plastic bags can cover plants. And that stops them from getting the vital sunlight they need. As well as micro plastics getting into the soil. This increases water evaporation drying out the soil. This also means that toxic accumulation occurs in the soil.