It is leaking out, because people make mistakes, or have a lack of attention to detail, or there is no liquid in the airlock
You CAN expect to see bubbles, but since there are factors that will allow pressure to escape by other means, the ONLY SURE way to tell if it is fermenting is with a gravity reading, because that is absolute.
This being said, in 5 years, mine have all bubbled.
Bubbling airlocks are not direclty tied to fermentation, only a drop in SG is.
Airlocks will also bubble if there is no fermentation taking place... they will also go in REVERSE evn if there is fermentation taking place. SG is the only indicator.