The chance of guessing a password is incredibly small without the use of password cracking technology. If you get even one letter, one digit, or one symbol wrong, you dont get in. If you use the wrong combination of lowercase and uppercase, you dont get in.
No, bad guys dont put their daughter's birthday or the name of the project they are working on as the password to their computer. They also dont leave hints lying around the base.
It would take a while to guess someones 4 digit combination much less any combination of words/letters/digits/symbols. And most of the time if you try to many times you get locked out. So how does one correctly guess a password with only limited knowledge on what it could be?
I see it as a very lazy catalyst to move the plot forward. Now I am not saying that all movies with password cracking scenes are like this, there certainly are some movies with a logical way for the protagonist to guess a password. But like 95% of the time it annoys the heck out of me.