Because most of vscode is not open source. See the python lsp, copitlot, plugin store, the fact that it's practically impossible to turn off telemetry/tracking. Gradually more and more of the actual value, is being moved into these proprietary systems and plugins to the point where vscode is really just an open source shim. In fact the vscode binary running on your machine does not have an MIT licence.

For now that's fine as it's mainly an on-ramp for Azure. But, if Windows is anything to go by, I imagine the enshitification of vscode is inevitable given enough time.