I can run a test someone hands me and report whether it passed. But designing one that actually catches the bugs, that finds the thing before a customer does, is a different skill I don't have yet. My tests go green and the bugs still slip through, and I can't figure out what I'm missing.
More people experience this than they realize.
You can run tests but you can't design ones that actually catch the bugs.
If this sounds familiar, the Library can help you find the bigger picture.