That's not really true. Most safety critical software is tested without formal verification. They are just really really thorough and rigorous.
Formal verification is obviously better if you can do it. But it's still really really difficult, and plenty of software simply can't be formally verified. Even in hardware where the problem is a lot easier we've only recently got the technology to formally verify a lot of things, and plenty of things are still out of reach.
And even if you do formally verify some software it doesn't guarantee it is free of bugs.
