If a SpaceX Falcon blows up on the pad, that's one thing. It's expensive but they accept that risk to move faster. At least they gain knowledge of what failed, to do better next time.
You can't apply that mentality once a human is piloting it however. That's how you get Columbia, Challenger, or Apollo 1.
Assuming it's not carrying a SpaceX Crew Dragon with crew onboard ;)
Also, it's a bit of a dated metaphor. Falcon 9 is by most accounts, now the most reliable rocket in history and is pretty design-locked. The modern metaphor is SpaceX Starship :)
The start is all rocket launches, which gives a good idea of how much is happening.
It was easy to find because they knew what valve was leaking.
It was not found beforehand because they don't have the ability to do the tanking test without rolling it to the launch pad and its very hard to know how a system responds to liquid hydrogen.