Because many Arduino sketches consist of only a single file, it's easy to start thinking they are all this way. However, Arduino sketches can consist of multiple files. In fact, it's the folder that's the sketch, not the .ino file. Each of the files of a sketch are shown in the Arduino IDE as a separate tab. All the tabs constitute the program. So it would be very confusing to show multiple separate sketches in the tabs, and the Arduino IDE is all about not being confusing. Having a separate window for each sketch provides a very clear differentiation between one program and another. So this isn't a matter of "live with it", it's a matter of the IDE being intentionally designed this way.
If you like having all your code in a single window, what you can do is use a separate editor program. There are many excellent options available (VS Code seems to be dominating that field at the moment). If you're using the classic Arduino IDE, you should open File > Preferences and then check the box next to "Use external editor". If you're using the beta release of Arduino IDE 2.x then that's not necessary because it picks up changes made to the file externally automatically. Then you can use the Arduino IDE only for compiling and uploading the sketch, using your favorite editor to write the code.