ArcGIS Pro has an environment setting called addOutputsToMap that is True by default, is NOT saved with the project, and is NOT available in the UI. If you do not want the output of every geoprocessing tool you run to be added to the current map, you have to run arcpy.env.addOutputsToMap = False in every Pro session, as it does not get saved with the project. Just because I want to make a bunch of new feature classes (or whatever the tool is) doesn't mean I want them in my current map.
Please make this an application-wide setting that is available in the UI. The Application, Geoprocessing options would be a logical place.