My only guess is that is that enchanting tables are mostly player created interactive items, and despite comprising of obsidian, would serve little real purpose to take a really long time to break. Imagine wanting to move it and having to spend all that time. It's also "just" a table as opposed to a full block, and is probably 3/5 of the Obsidian content that an actual block is (Wiki says it's 3/4 of a block height-wise as far as the game is concerned, but depending on how much room they take up, the Red covering and the diamond corners may reduce that a bit more, so I'm roughly saying it's 3/5 the content of a full block). So, despite needing 4 of it for material cost and to make sense in the crafting grid, it's less in the finished product. Why? No idea. You'd have to have 1 Obsidian block break down into 4 "lesser" types of blocks (think logs into planks) to have it make sense but that'd serve little real game-play purpose. It just needs 4 blocks for the cost and to make sense in the grid.
Similarly, the Ender Dragon can break this block, despite it being made of Obsidian. Minecraft is nonsensical like that.