At the time those cinematics were top tier. Blizzard continued to have some of the best cinematics until Warcraft 3, after that I believe they and many other game developers switched to in-engine cutscenes because the engine visuals were good enough. That said, Blizzard does still make prerendered cutscenes for Diablo 4 and WoW, but they're just... not as impressive anymore as they used to be back then. And a big part of that is that they're not that much different from in-engine. There's some games (FFVII Remake/Rebirth) that have pre-rendered cutscenes that are visually indistinguishable from in-engine cutscenes, but they pre-rendered them because of e.g. wider / zoomed out camera angles or lots of effects.