Lots of reasons ... inadequate training, inadequate knowledge, inadequate comprehension, inadequate experience, inadequate awareness ... or a simple dose of denial.
Because agencies don't teach proper gas management, many (most?) recreational divers tend to think more about how much gas to end the dive with, rather than consider before the dive whether they have adequate supply for the dive they want to do. Many (most?) have no comprehension of their "MPG", and how depth will affect it. Divers often tend to go deeper than is wise for the gas supply they brought with them (because they don't know any better, and the DM said it was OK). They don't realize how things like narcosis, current, wildlife, or simply the excitement of being on vacation can increase their air consumption and reduce the time they thought they had available for the dive. Or they simply get caught up in the scenery (often viewed through the lens of a camera) and forget to pay attention to their gauges ... either the SPG or the bottom timer ... until they're past the point of no return.
I think the majority of gas issues are not OOA, they're LOA ... people suddenly realizing they don't have enough to make a safe ascent, although they still have some reserves to breathe off of while they figure out what to do about it.
Today's fast-track education may be well-and-good (depending on how you look at it), but the one thing it cannot do is improve people's mental bandwidth. The vast majority of divers have to consciously think about the basics of what they're doing ... and are operating on such slim mental margins that a simple distraction will cause them to neglect something really basic ... like checking their gauges.
On the opposite end of that spectrum are those experienced divers who have developed an inflated sense of their abilities ... and are prone to complacency. They have even less excuse ... although they've had time to come up with some really creative ways to rationalize their errors.
... Bob (Grateful Diver)
