It is inherited from the Eclipse Theia IDE framework the IDE is built on, which inherits it from VS Code.
So it is not something Arduino designed, other than through the decision to build the IDE on Theia.
I'm certain that Microsoft gave a lot of thought to this UI design, and it seems to have been well received since VS Code has become the world's most popular IDE. Of course it is true that the target user of VS Code is significantly different from the target user of the Arduino IDE.