Why are London forces weak?
The whole basis of London dispersion forces are temporary dipole moments that induce the same in other molecules of the same species. The very nature of these intermolecular forces are fleeting, as opposed to permanent dipole-dipole attractions, and at the extreme of the latter—hydrogen bonding.
Paraphrasing my organic chemistry text:
To be sure, consider that on average the center of negative charge coincides with the center of positive charge. However, at any instant, this may not be true. As a result, a transient dipole moment is created that induces another in a neighboring molecule (Klein 2015).