Computer UIs needed borders and outlines because there are no brain-intuitive visual cues: no depth parallax, no shading, nothing shifts as you move your head, and until relatively recently they had poor contrast and brightness variability compared to the real world.

It was also a compromise for interface device limitations. We didn't have 4000 DPI mice with scroll wheels and 26 configurable buttons; you were lucky to have a 1024x768 resolution; and 16 bit color was for people shelling out $$$. Obvious borders and some padding between elements were a necessity to click what you intended to click.

Of course real life has outlines. Look at these for instance:

https://www.reddit.com/media?url=https%3A%2F%2Fpreview.redd....

https://hackaday.com/wp-content/uploads/2026/05/skala_displa...

Displays & controls are thematically grouped and/or framed, each button/rotary/switch has a color that constrasts from the background, and there are strong 3D clues to see what's a button, and what's a label or the background.

You will never see a visually consistent operating system again, the material conditions that allowed for older Windows and Mac OS versions to be consistent no longer exist. The user base for desktop computers is no longer growing, so the investment into new desktop UI technologies is largely in technologies like Electron and SwiftUI that allow developers to cut costs by reusing their existing web and mobile interfaces for desktop software. The focus on cross-platform development means that developers are far more concerned about making their software look consistent across all platforms than they are about whether their software looks native on each platform it supports.