In some cases I guess there could be a large suite of regressiontests that will find "all" bugs and verify the behaviour, thereby allowing for a quick and dirty coding technique. But mostly its just a matter of bad project planning and a manager who thinks its more important to get it done, than to get it done right.

And forget about "clean it up later", that never happens. In the rare cases it happens, the programmer will have forgotten most of the code making the job alot more expensive than if he had done it right the first time.