Comment by MrGLaDOS

Arecibo and FAST are giant immovables dishes (essentially a parabolic valley covered by some electrically conducting material). The receivers suspended above these dishes are somewhat moveable, but that is about it. That means they can only observe a small region of the sky exactly overhead (this region changes together with the rotation of the earth). VLBI requires multiple telescopes significantly separated from each other in physical space (think thousands of km) to track the same object on the sky during an observation. Thus VLBI is hard/impossible for giant immovable dishes like FAST :)