Why do they do this?
Many of the problems arise from the fact that
students use common sense rather than reason
using the physical principles they have learned.
Students use their natural and spontaneous responses
based on experience and overly simplistic reasoning.
(I know how light [or motion {or electricity}] works.
I dont need to go through that confusing physics stuff
to get the answers.)
Most students do not spontaneously seek to build
the tight consistency and coherence required
by a scientific approach. It needs to be learned
(and taught).