Singular "Plastic" doesn't exist, we use several hundreds of different plastics for many purpose, each of which having its own requirement (sometimes it's its lack of biodegradability, but sometimes it's its transparency, or its light weight, or its elasticity, etc.), each use case would need a totally different substitute.

In all cases, though, a key feature is that it can be synthesized at massive scale for cheap, and it's the hardest part when looking for substitutes.