system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: We have invented materials that don't rust, so why do bicycles still use chains that turn rusty after a little rain?
gpt
Materials selection is often balancing a set of trade-offs. A chain needs to be both durable and relatively easy to make.

Using a hard material and adding lubricant helps it last a very long time, and that lubricant can also prevent rusting if properly cared for.

Very few materials are as hard and durable as steel, and those materials tend to be super expensive.