Distractions, other ideas, other opportunities, failure, negative self-talk, etc, would get in the way.

And the results are always the same: You never get anything done.

To me, the key finding from the study by Dianne Tice and Roy Baumeister is this:

"The present evidence suggests that procrastinators enjoy themselves rather than working at assigned tasks, until the rising pressure of imminent deadlines forces them to get to work. In this view, procrastination may derive from a lack of self-regulation and hence a dependency on externally imposed forces to motivate work."