Because we assume that developers are trained professionals, presumably with a CS or software engineering (or both) degrees, and that they've been properly trained in software development - which puts testing front and center.
If by 'testing' you really mean 'unit testing', as I suspect most junior engineers who claim testing experience do, then hope is already lost. The one saving grace is that there is enough churn in webdev that nothing lasts long enough to reveal how fragile it is.