To answer OP's question, they hired some Snap-on Engineers away from Kenosha. Have to pay for that extra overhead somehow.
Now, as far as sheet metal goes, it all comes down to the gauge, number of folds, and welds. That determines how rigid the box will be and what type of repetitive stress it can handle. After the raw sheet metal comes the finishing process. The cleaning system and prep before paint, followed by the actual paint (powder) itself. Not all cleaning, pre-treatment and powders are created equal. Then we move on to purchased components like slides, handles and casters. These items should be self explanatory to anyone that uses tools for a living. Break every step of the process down from coil stock to your garage and you will see why there is a difference between the box store and tool truck boxes.
The debate to justify the extra cost comes down to your personal opinion. Nobody needs anything better than a Casio G-Shock to tell time, but Rolex isn't going out of business anytime soon. Deals can be had on the high end boxes with some patience and cash in hand. You are still going to pay more, it's your choice, we live in a great country that allows us to argue about little shit like this.
Disclaimer: I have a tool addiction problem, my wife only allows it because she hasn't had to call a repair guy or a mechanic in the 20+ years we have been married.