It’s evolutionary pressure; software is malleable and potential functionality is limitless. Software companies that didn’t ascribe to this philosophy were repeatedly killed by ones that did until it became the status quo.
I mean, that's rather disputable: The Apollo 1 exploded, medical mistakes have a toll of 250.000 deaths per year in the US alone; among many other serious mistakes on vast different areas, I think unreliability is unfortunately a constant on the human race.