Probably because a fixed seed is also more resource-consuming. For all intents and purposes a container is dead to the game as long as it's unopened. Only when it is, it will become a part of savegame content. And it can be completely wiped if there's a path of no return (though I am not sure if Original Sin will be like that). With a seed like yours ALL containers will need to be logged from the start. Increasing loading times, saving times, savegame space usage, etc.
No, there are other ways to handle it. Just to name a handful of examples, most containers can be kept as is, while only specific special-case containers are handled differently. You can also populate these as the player enters a specific region (which could be a specific building, a dungeon, etc). You can also seed based on something specific (something about the characters data), which would give you the same results from the same container regardless of reloads, unless you returned to that chest at a much later point when substantial changes have occurred with that character.
The concern about lockpicks I pointed out stops being a concern if vendors start selling lockpicks at a reasonable price (which is a change that should happen anyway).
