For Rider, it's because they built ReSharper (a Visual Studio extension) before Rider, and then built Rider to make it use ReSharper as a back end without Visua

For Rider, it's because they built ReSharper (a Visual Studio extension) before Rider, and then built Rider to make it use ReSharper as a back end without Visual Studio, while still using the IntelliJ platform.