Duct leakage is due to several factors: The primary cause of duct leakage is that the connections were never sealed when the ducts were first installed. (Remember, back when energy was cheap, plentiful, and nobody really cared about their utility bill) There is often damage or separation when the house settles over the years (often crushed or separated pipes from 3rd parties doing work in the attic).
Also, the ducts are usually only insulated with R-3 insulation which results in a significant amount of heat gain/loss as the air travels through the ducts in your attic or basement/crawlspace. When we foam seal old ducts, the insulation is upgraded to an R12.