Natural rubber (latex) is biodegradable, but that doesn't mean it isn't bad for the environment. The production and disposal of natural latex causes all sorts of problems unrelated to microplastics.
Synthetic rubber is chemically distinct from plastic, but still breaks down into microplastics.
Natural rubber tires are vulcanized, which makes the rubber more resilient, but also more damaging to the environment.
What's crazy is that we could do soooo much better, but it would mean rich people would make less money. Not no money, just less of it.
Nothing needs to be made of plastic. It's cheap and convenient, which just means "profitable." If we didn't use plastics, we'd use something else and it would be less profitable.