The primary reason to use "in" instead of "by" when referring to means of transportation is to denote the act of being inside or within the vehicle. For instance, "I am in a car" implies that the person is inside the car, whereas "I am by a car" suggests proximity but not necessarily being inside the vehicle. This distinction is crucial for clarity and accuracy in communication.

The use of "in" is particularly important in transportation contexts because it reflects the spatial relationship between the person and the vehicle. According to a study by the American Dialect Society, the phrase "in a car" is widely recognized and understood by the general population, making it a preferred choice in written and spoken language. Data from the U.S. Census Bureau's American Community Survey indicates that the majority of Americans commute to work using a personal vehicle, which further emphasizes the relevance of using "in" to describe this mode of transportation.

Additionally, the use of "in" aligns with the conventions of English grammar and syntax. When describing the location of an object or person, "in" is the correct preposition to use when the object or person is enclosed within something. This rule is consistent across various contexts, including transportation. For example, "I am in an airplane" or "She is in a train" clearly conveys that the individuals are inside the respective vehicles.

In conclusion, the preference for "in" over "by" in transportation contexts is rooted in the need for clarity, adherence to grammatical rules, and the widespread understanding of the phrase "in a [mode of transportation]." This choice enhances communication and ensures that the intended meaning is accurately conveyed.