"The game was really designed from the get-go to be about [killing and looting]," Diablo designer David Brevik said in an interview with Kotaku, "Our intent wasn't really to pry money from your wallet. It was more that we liked the addictive feeling of just one more monster, just one more piece of loot."
Diablo and its sequel weren't designed around story and characterization as most RPGs are - instead, according to designer Brevik and Erich Schaefer, it was all about treasure troves and skewering demons. Since then, loot has become somewhat of a controversial topic given the advent of its evolution into microtransactions and loot boxes that require real money rather than the slaughtering of enemies to acquire.
"I don't feel too guilty myself," Schaefer opened, "I don't use those systems myself in games I've made. But anything can be exploited for good or evil. I don't feel like we shouldn't have unleashed loot upon the world." Ultimately, it's up to legislators and individual developers to avoid the issue of implementing addictive schemes that have been compared by various organizations and governments to gambling.
That being said, Schaefer compared the looting of the original two Diablo games to slot machines, where slewing foes was akin to pulling the lever and hoping for something good to come of it. It built up that hope for getting a decent drop, and when you'd go through the killing of so many pawns that leave behind mundane, uneventful 'treasure' in their wake, the explosion of gems from a boss suddenly slams you with a wall of dopamine.
That's what loot boxes are or, at least, what they do - you open them up, hoping for something good. More often than not, what you find is a series of white, 'common' gear, be it currency, crafting materials, recolors, sprays, etc. When a golden legendary with eventful music to boot splashes out onto the screen, it's a cheerful, celebratory moment. The big difference is that one is in a video game where you play to get that moment, no extra cash required, and the other has you forking out the big bucks over and over and over until you do get that good loot, emptying your wallet dollar bill by dollar bill.