The dungeons are older and often an even bigger mess than the leveling content though. They are struggling to even find ways to keep casters relevant in low level group content. You go into these low level dungeons and it's rare for it to seem like there are even mechanics. You get stuck in this very repetitive combat loop that you apply to almost every pull. You pop out of the dungeon momentarily, or sometimes not at all, and find yourself thrust right back into repeating that same loop.
Art assets aren't holding people back from leveling through older zones by questing. Travel time between objectives is high for little to no return when weighed against being transported to a dungeon with new quests laid right in front of the player. World of Warcraft's leveling is fundamentally broken from a numbers perspective, which is why we see them finally going in to try and tweak that stuff. They're still far off from setting things on the right course though.