If your company takes months to figure out if someone does good work or not then it has some serious problems.

If you have expertise in a field you can normally guess pretty quickly when someone bullshits their way through or actually knows what they’re talking about.

But your hiring staff usually don’t have any or deep enough expertise for the field they’re hiring for, which only gets worse the bigger and more advanced your company gets.

But your employees with which the new hire will be working with should have this expertise (or again your company has a problem). So they will find out pretty quickly if the hire is a fraud or not.

Cheating surely creates costs but not as much as you suggest. And as others have mentioned above it is surely not as widespread as you suggest, limiting the cost further.