It's a good question. The cartoon says that qubits are "unit vectors in 2D Hilbert space." However because the length is 1, you can think of it as a vector in 2

It's a good question. The cartoon says that qubits are "unit vectors in 2D Hilbert space." However because the length is 1, you can think of it as a vector in 2D projective Hilbert space. Projective space is what you get when you throw away a vector's length and only talk about its angles. Some intuition: if you have a particle and you look everywhere for it, your chance of finding it must be 100%. It can't be 50% (where could it be?) and it can't be 200% (duh). QM in one dimension is formulated as "a 1d complex unit vector", i.e. a complex number of magnitude 1, which is routinely represented in polar form (a complex exponential). QM in two dimensions is a "2d complex unit vector" which is really 3d: you get three linearly independent components, and then the fourth one is decided for you. You can think of this as spherical coordinates, but complex numbers are much easier to manipulate, so that's what we do.