Why do the Transformers always transform into the same cars?

The Transformers aren't very well explored in the Michael Bay movies, so we'll have to judge according to the other (better explored) continuities.

While the process of switching alternate modes in the movie seemed effortless, it might not have been. In other continuities the switch is either energy consuming or a major physical operation. So just changing to another vehicle might be a very expensive step that one does not take lightly.

Also, the alt-mode seems to be a very personal aspect for each Transformer, which serve a similar function as clothes do to humans. They are a reflection of the owner, and once it is chosen it tends to stay the same. Transformers have been known to keep the same alt mode for thousands of years, which isn't really that long when you remember that their life span can stretch into the millions of years.

The first thing to remember is that the Transformers are a toy, and for a toy multiple transformations are not possible. The films also used the same old transformation modes for the characters from the franchise.

In the films, their transformations are not well explained, but in the animated series and related shows, it's explained that Autobots used to transform into public vehicles so that they can mix with real vehicles, but Decepticons used to be transform into fighting machines.

But their transformation is not that easy. To become something else, they have to upgrade to them. In one animated storyline, Optimus Prime became a jet by merging with an Autobot.