I am also not a mathematician, and indeed got fairly mediocre math grades throughout most of my education. But you don't need to be PhD candidate to understand that if you never actually check, you never actually know.
As we all recently discovered - not testing means the thing isn’t happening (obviously), which is convenient if you don’t want to know.
Some people go looking for problems, and they find them. Some people don’t, and they don’t. Some people are so uninterested in finding problems that they’ll attack or ‘fix’ people that find them.