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.