Why do presidential candidates believe they can pass laws when that is Congress's role?4 of D 5 recent Presidential candidates served in D...

These candidates of course know that they cannot address
these issues on their own.  They know that only Congress can actually pass laws.  The
candidates also know that a role of the president is to set the agenda for Congress and
to show leadership in trying to get Congress to pass the laws that they (presidents)
think are needed.

A major role of the president is to lead
the country in the direction he (no women yet) wants it to go.  Presidents and
candidates know they can't do this by simply issuing laws on their own.  They know that
they have to suggest laws and try to persuade Congress and the public to support those
laws.

Only the president can really do this effectively. 
Most Americans do not know any of the leaders of Congress.  They would not pay much
attention to the Congressional leaders speeches and such.  But when the president
speaks, it is big news.  Because of that, the president is much more able to lead the
country because he has what Theodore Roosevelt called the "bully
pulpit."

So, these candidates know they can't rule by
decree.  But they know that it is the president's job to try to lead the country and the
Congress in the right direction.