The same way one explains that mental arithmetics are useful, even though calculators exist.

“You’re going to have to maintain that code.” (Or maybe: “You show great promise as a dev, but one day, someone less skilled than you is going to have to maintain it.”)

“Are there any security vulnerabilities?”

Edit: But in general, this sort of thing happens during any transition. I suspect that engineers double-checked what CAD output when CAD was new, and the kids-those-days just trusted it, can you imagine?