Transistors have some nuance to them, but a transistor can in theory be scaled all the way down to being made out of only a handful of atoms if it was made perf

Transistors have some nuance to them, but a transistor can in theory be scaled all the way down to being made out of only a handful of atoms if it was made perfectly with no impurities, and it would still function nearly identically to the slightly larger versions. Of course, packing them tighter still has some problems due to quantum tunneling at the scale of handfuls-of-atoms, but that's a separate issue. Meanwhile a brain cell is still a lot more complicated and has a place in an entire network of cells and the connections between them that cannot be described by the comparatively simpler equations one can use for a transistor.