WordPress needs themes and plugins to be in web-accessible folder, so that any scripts and styles from them can be downloaded by browsers and work.
From there it’s technologically easy to detect such resources in page source and/or proactively scan site for presence of files belonging to popular plugins.
There is nothing to be done about that, short of massive layer of moving and anonymizing any files that need to be loaded. I am not aware of any solutions the purposely do this. Plugins that concatenate scripts and styles can have partial side effect like this.