For a lot of applications plastic is also simply a really good material. It is cheap, easy to form, strong, can be made either rigid or flexible, and has a low CO2 footprint. When it comes to inherently single-use applications it is incredibly difficult to beat plastic, and even for multi-use applications it often doesn't look great for materials like metal or glass.

And yes, littering is the main downside. I agree that recycling is mainly bullshit, but you can also burn it together with the rest of the trash, and get some free electricity from it!

Microplastics are indeed a serious problem, but it is almost entirely unrelated to day-to-day plastic items. The primary source of microplastics is car tires, with synthetic clothing being a large contributor as well. And there's of course the absolutely insane use of plastic particles in personal hygiene products. If you want to reduce our microplastics exposure, those are probably better places to start.