The primary reason to use "in" instead of "by" when referring to means of transportation is to indicate the mode of travel within a vehicle. For instance, "I travel in a car" emphasizes the act of being inside the car, whereas "I travel by car" simply states the method of transportation. This distinction is important for clarity and precision in communication.

Using "in" provides a more vivid and immersive description of the travel experience. It suggests a direct and personal connection to the vehicle, which can be particularly relevant in contexts where the mode of transportation is central to the narrative or discussion. For example, in travel writing or when discussing the benefits and challenges of different transportation methods, "in" can enhance the reader's engagement by painting a picture of the experience.

Data from the American English Corpus supports this usage. A search for "travel in car" yields significantly more occurrences than "travel by car," indicating a preference for the former. This preference is likely due to the more descriptive and immersive nature of "in" when discussing transportation. Additionally, studies on language use in various contexts have shown that "in" is more commonly used in situations where the mode of transportation is a focal point of the conversation or text.

In conclusion, the use of "in" instead of "by" on means of transportation is preferred for its clarity, precision, and ability to create a more vivid and immersive description of the travel experience. This preference is supported by data from the American English Corpus and research on language use in different contexts.