Lazy writing - that's why it happens.

Now, nobody wants to watch a movie that's 100% realistic, unless it's a documentary. If I want realism I'll step out of my door. Nobody goes to a horror, fantasy or science fiction movie expecting a science lesson. BUT the story should at least be internally consistent.

and not outlandishly incorrect ...
you cant get involved in a movie if it is glaringly innacurate ..especially when dealing with science ...

but i agree .. that has a threshhold - we're not all Stephen Hawking