Why is it a bad idea to divide reference voltage only with resistors instead of using an op-amp buffer?
Because "for best performance, the source impedance to the REF terminal should be kept below 2 [ohms]". 

If you made a resistive divider to meet that requirement, it would be 4 ohms in each arm, or 8 ohms between power and ground, which would likely consume a lot more power than you'd want to use for this function.

Why does the impedance need to be below 2 ohms?

It's explained in the text you posted: