As AerocatS2A has mentioned improper planning and poor situational awareness are key components of unstabilised approaches. Aiming to be stablised at 1000' give little room for errors in what is very dynamic environment filled with vast amounts of variables.
But this does answer the why question?
sure we can quantify the approach phase and sum up on how the approach unfolded but more often then not the underlying cause is still left unanswered, why crews neglect the stabilised approach criteria and still continue the approach <1000'.
look at it from a cognitive point of view, as human were a tendency to be loss averse (loss aversion) which refers to people's tendency to strongly prefer avoiding losses to acquiring gains which in tern may prompt us to take on more risk. Loss aversion vs Risk Aversion theories and decision making are well documented within the investment and economics domain.
put simply regardless of the sequence of event leading up to the 1000' gate.. what prevents the crews from going around....uncertainty...
- go arounds are not conducted on a frequent basis hence reluctance initiate once can be borne.
- Operational consideration (duty, time, connecting transit pax, next sector departure time..etc)
- WX
- Fuel
- and others
- ATC, traffic
- other i can think of right now
look at it from this point of view .. we are trained to manage an aircraft with a high degree of technical ability yet most training systems don't teach the individual to manage or improve their decision making process... and untill that happens unstablised approaches will continue to happen more often then it should...