I’ve only ever used Outlook when forced to by an employer and I find it a dreadful application to use. I would guess that most people prefer something else. I would imagine that most people tend to stick with the default email app on their computer (no idea what that is on Windows as I’ve managed to avoid having to use Windows for 7 years now).

The default mail app on Windows is now called Outlook for Windows, no relation to the Outlook in Office (sorry, Microsoft 365 Copilot), and it's a significantly worse barely functional webview. It also replaced the entire Calendar app, which was decent.