I have noticed that leaders often say what they believe their folloers or constituents want to hear.  This is not just politicians but successful Corporate and other group leaders (e.g. religous, charitable, etc.)

Human nature, the basic framework for average thinking processes is architected to allow for us to pull the wool over our eyes and believe it.

Desirable or Possible

My own believe is we should set our objectives based on the possible.  That is the high level goal should be analyzed for feasibility together with how long and how much effort is needed. (part of the feasibility).

Most often the accountable leader constructs the doability of an objective around the perception it creates.  That is, they say they will do something not because they know it can be done but rather because they beleve it makes them look good.

Conflict of Interest

The people responsible to execute the leaders vision and objectives are often frustrated because of the lack of buy-in.  Both theirs and the leaders, one thinks it can be done and the other not.

This dichotomy can lead to confusion and is a underlying cause of why goals often take longer to achieve than original publicized.  Both because of lack of buy in/support and because the leaders pronouncement have no foot in reality.

Instant Gratification

Our culture, particularly corporte and political environments, are founded on the concept of immediacy.  For many, long term strategies have no merit.  

I want it now is the primary thoughtprocess and therein lies the problem.  Changing cultural beliefs, so all understand the value of being real even if it sounds like it is going to take longer.

We can try to change the thinking but most often we just struggle with the unrealistic.