Question

Radio waves are able to diffract readily around buildings, as anybody with a portable radio receiver can verify. However, light waves, which are also electromagnetic waves, undergo no discernible diffraction around buildings. Why not?

Answer 1

diffraction is noticable for long wave, not for short waves.

Answer 2

Here are few additional comments to Bob Pursley's correct answer

Waves spread sideways a distance x beyond edges of objects by a characteristic distance
y = sqrt(wavelength*X)
This is called Fresnel diffraction

There is another type of diffraction in the "far field" past objects of size D that block waves. It is called Fraunhofer diffraction and has a characteristic spread angle
theta = wavelength/D