Sometimes what we perceive as bad dialogue is actually bad translation of dialogue that may (or may not) have sounded much better in its original language. Having worked as a translator myself, I can tell you it's not just a matter of matching words - one has to be able to get across lots of additional information, some of it implied by things like word sequence and punctuation.
Another point to consider is that dialogue in games serves more functions than in movies. The movie takes its course whether you heard a conversation or not, but in a game, missing a piece of dialogue can lead to your missing a clue and, later on, being unable to proceed. This is why game dialogue sometimes appears badly written - it has to repeat important parts, it has to give clear and precise instructions, etc.
Just my two cents.