There are several ways different games deal with loot.
Static drops: specific chests always contain the same items, specific enemies always drop the same items when defeated. Period. Many older RPGs do use static drops.
Random drops: as in, completely random. It doesn't matter what you kill or where you are, every/any enemy has a chance of dropping any/every item. Some ARPGs lean towards this method.
Most games fall in between those extremes, using some combination of both. Some lean more one way or another. Diablo 3 for example is very close to pure random, but there are a few things that are limited to specific actions (act bounty specific items/mats for example are exclusive to those events, but most everything else is completely random from anything/everything). While many older RPGs used purely static drops for most things, but could still have some minor random elements. Baldur's Gate 1, early Dragon Quest and Final Fantasy games, The Legend of Zelda, for example, would have mostly static drops: specific chests would always drop the same specific items, specific bosses would always drop the same thing, but random encounter enemies might have their own very small loot table and would have a chance to drop consumables (ie: goblins have a 20% chance to drop a regular potion when killed, while a red dragon has a 5% chance to drop a Red Fang that casts Firaga when used as an item, as a random example)
DDO is also a hybrid system. There are some items that are guaranteed (via specific end rewards lists), but most items are tied to specific chests/quests, and you have a chance (random) to get said specific item from that end chest, however it must be noted that they are tied to that specific quest/chest, so you can target farm them. You want a specific item, you go to a specific chest that can drop it. As mentioned, some named items always appear in end rewards lists. Some named items appear every 3rd end rewards list (guaranteed static drop after 3 tries, random drop every other try). Some items can be worked towards via runes, and some raids still have a 20th completion list as well. Not every quest/item is a part of the semi-static drops, and are always random drops, but are still tied to specific quests/chests. A very very few items are completely random and have a small chance to drop from any chests. Then there are the plethora of crafting systems (not all are tied to raids either) for customization.