Multiple causes of not reading instructions
As @crash notes, there are likely many explanations for not reading instructions.
It may be motivated by not caring about task performance. And such dispositions may be specific to the particular task or setting, or they might be partially related to some general disposition of the individual in terms of conscientiousness or skill.
The skill of not reading instructions
However, more generally, the concept of information reduction is related to skill acquisition. Haider and Frensch (1996) discuss the idea of how gradually over time participants start to ignore elements of a task that appear to be unrelated to task performance. So on one level, learning to ignore irrelevant information is a skill.
With regards to instructions, it is not always the case that reading an instruction manual is the most efficient means of achieving your goals. Trial and error, exploration, and so on are other strategies which often work quite well. In particular, many devices and software these days are designed to encourage people to explore. Likewise, it can often be a waste in learning information that may or may not be relevant, and so in some cases it makes sense to access instructions on an as needed basis.
It is also one thing to sit in the position of all knowing observer where you can know that in a particular case it would make sense to read the instructions over using an alternative strategy. However, from the actors perspective, they are in a sense operating without that knowledge. So they are in a sense averaging over their experience to form a decision about whether or not and in what detail to read the instructions. In some cases this will work out for the best, in others it wont.
General problem of task design
If you know that reading the instructions is particularly important in a given domain then you may want to alter the system or the task to encourage reading the instructions. Many organisations have compulsory training with a test in order to force users to learn certain information. Alternatively, you could make clear why reading the instructions is particularly important for this task. Similarly providing strong negative feedback when task instructions are not followed may help the individual to learn that in this domain reading instructions carefully is important.
That said, it's often even better if you can incorporate human factors principles to design a system which is user friendly.
- Haider, H., & Frensch, P. A. (1996). The role of information reduction in skill acquisition. Cognitive Psychology, 30(3), 304-337.