The simple answer: online lacks all the visual and aural feedback mechanisms that are automatic in face-to-face interaction. That radically changes how people act.
All the social norms that have 100,000 years of biological and sociological development that are kept in-check by visual feedback from others around us are taken away online.
This also applies to things like sales - face-to-face direct sales is far more efficient and effective than doing the same by phone or online. You can close deals far higher $$ amounts face-to-face because people can "read" each other. Online works just fine for simple, low $, low risk transaction - that's Amazon's sweet spot, for instance.
I've been in high tech sales for 30 years - with early experience at HP selling test equipment with average selling price (ASP) of $20K-$1M. I'm still in that general market. The sales training you get at Fortune 20 companies addresses most of this difference both directly and indirectly. The latter in includes noticing how direct sales works and what can be sold that way vs. what can not with telephone and online sales (the risk level of $1M piece of equipment even for the largest corporations absolutely requires a lengthy face-to-face process).