Why Senior employees think they would take more efficient decisions to improve work environment and enhance workforce performance than leaders?

Senior employees usually know more about the details of the decision to be made than the managers do, because they are closer to it.   Bad managers are often out of touch with the current realities of the situation.  

Managers are often constrained by higher level aspects of a decision, which may be confidential or unappreciated by the senior employees.  For example their may be political or strategic considerations.

One of the traits of a true leader is that he or she is aware of these aspects of decision making, and will act accordingly.  For example, when a good leader knows that there are no higher level considerations and that the senior employee really does know what he or she is talking about, then the response may be something like, "You decide, and I'll back you."

It is a misconception that a good leader has all the answers.  Good leaders know their limitations and they set goals, coordinate, motivate, mentor and support their personnel.  They will therefore allow the most experienced personnel to make the technical decisions during the execution of a project or task.

That is why in most successful companies the leaders are not necessarily technically qualified and experienced personnel - think of Jobs and Apple - but they lead the technical experienced and qualified people to be a coercive and productive team to achieve the set targets.

Therefore senior personnel would probably be right to believe they know better to get the work done, but it will take the leader to set strategic direction and coordinate their efforts.

There are, however, cases where the leader also have to be technical expert - such an example is a Professor at a University, a Medical Specialist, etc.