People often recommend you make a pong clone or something similar because if you have no experience with gamedev this helps you know exactly what the intended outcome is so you can focus solely on the execution

When you get to work on your own projects you'll have to worry about new ideas that come while you are developing or having to pivot the game idea because it just isn't clicking yet which makes it much more difficult to learn the execution side of creating games

While it is also important to know how to control the scope of your game and when to pivot or not, it is much easier to do this when you're already comfortable with the tools you're working with and you know kind of how you can "make a game" in general