Why can't we have an IDE that provides a "single file" view into multiple files? It would display them one after the other in a single tab, allow search across

Why can't we have an IDE that provides a "single file" view into multiple files? It would display them one after the other in a single tab, allow search across multiple files, probably even cut-and-paste from one file into another without switching tabs. And of course it would be easy to choose which files go into this view - you could have all_models one moment, models_and_controllers_related_to_feature_X a moment later, without changing anything in real files. Wouldn't such IDE feature provide all benefits of a single huge file - but with different physical files, e.g. for source control purposes?