Yesterday I got pulled into a layer-2 DCI tweetfest. Not surprisingly, there were profound opinions all over the place, including “We've been doing it (OTV) for almost a year now. No problems.”
OTV is in fact the least horrible option – it does quite a few things right, including tight control of unicast flooding and reduction of STP scope.
You might as well ask why people insist on not wearing seatbelts after all of the years that particular technology has been proven to save lives.
People will, it seems, persist in the optimistic belief that everything will be OK so long as they are otherwise careful. They think that bad things happen only to other people’s protocols, or packets, but not to theirs. Hope springs eternal and dies in the cold, cold winter of experience.
Finding this one a day after discussing layer-2 DCI? There really are no coincidences.