Same reason why a carpenter should learn to properly use hand tools, or a teacher should keep up on literature. In other words, master your tools. It doesn't matter which tool you use, provided you can use it to its full potential.

I occasionally use VSCode, but I mostly use ViM because that's my tool of choice.

For another reason, if your ever in the situation where you need to work on something on a remote server or an unfamiliar machine, knowing ViM means you can at least be somewhat productive when you don't have your normal tools available.