Engineering is not only about creating robust designs, but is about creating a design that meet some specifications. Usually young designers don't fully understand that economic factors *are part of the specification*. The problem is that sometimes those economic factors are not always well specified (that's often a management's fault), but a good designer is somewhat expected to consider also non-strictly-technical aspects in his designs, such as:

+ BOM-related cost: *who cares if 1% of the units fail in the field if it is more economical to ship a new one to the customer instead of making all of them more reliable!*
+ Time to market: *who cares if the units are more reliable if our competitors ship their things one month in advance!*
+ Planned obsolescence: (sad, and not environmentally friendly, but usually it goes like this): *why would we ship a unit that can last for 20 years if we marketed it to be able to withstand for 5 (and we made a lower price point for that)?!?*
+ etc.

All this depends on the field where the design you are creating is targeted, of course. If you aim at a market where a single failure could cost lives (say a new defibrillator), you will apply more safety margins to your design (and will be also forced to do that in some cases by mandatory safety standards).

Stricter specs are good if, for example, you are designing a mission critical board for a space probe for a ~1M$ mission to Pluto. In that case you really would want to foresee the unforeseeable and test for any darned little thing that can go wrong. But this is counterbalanced, economically, by the risk of being sued (or fired) by NASA because your crappy MCU code made all the mission go awry!

To recap, experienced *successful* designers know how to manage all these economic factors. Of course some of them are really smart and really understand all the delicate balances needed to bring a project to success (be it the new Apple iMostUselessMuchHypedphone or the best instruments for detecting bacteria on a comet). Some other, incredible but true, are just lucky and find the right niche where the "Does the prototype work after being mistreated a bit? Ok! Let's ship it!" mantra works well!