Knowing lots of facts without anything to unite them is like having a crappy Tetris board.
Without a mental model to unite what you know, you have lots of isolated blocks that don’t fit together well and you can’t clear the board. You have no room for new concepts, and your foundations are full of holes that can’t be filled in.
Unifying concepts are those nice rectangular blocks that let you take an entire row of knowledge and reduce it to nice empty space, ripe for new knowledge.