[robocat]

Since the wavelength of 60GHz is approximately 5 millimeters, this technology is sometimes referred to as millimeter-wave (mm-wave). (Copied quote).

That explains how close together the antennas are - close enough compared to wavelength to be able to beamform.

Edit: also explains why it would be extremely difficult to build something yourself at 60GHz - where every wire needs length to be matched to submillimeter length, and a submillimeter tail acts as an antenna and as an electronic component.