It is great to live in an ideal world, and in fact, in most software, you can do what you are suggesting quite cheaply. But once you get past the sort of "quip on hacker-news" level of thinking about this, or trivial and cheap production testing scenarios, people have to make real tradeoffs because it's never that simple.

Talking about those is much more interesting than just asserting that everything should be a certain way, without any consideration for real world constraints, like cost of units, etc.