Neither VS Code nor IntelliJ come with full SDKs for macOS and iOS (and in the case of Xcode 11.5, also tvOS and watchOS), nor do they come with simulators, or even basic (let alone full) development toolchains at all. You have to install those separately with VS Code and IntelliJ. You "only" get an IDE.
When looking at Xcode 13.4, for example, with a tool like DaisyDisk, you see the bulk is the iOS simulator: it alone weighs in at over 8GB and the bulk of that is the iOS runtime (the "operating system", if you will). Of the remaining about 5GB of Xcode, a lot of that are the toolchains (compilers, linkers, various other tools) that you would not count with VS Code or IntelliJ as you have to install them separately.
Xcode is not just an IDE. It's a whole bundle of tools and resources you might need to develop for all of Apple's ecosystems. It has the upside of having a lot of tools and functionality you might not even have heard of yet out of the box, and they're tuned to work together. The downside is the size requirement.
