I agree that native speakers aren't choosing words based on whether they are shorter and thus quicker to say. As Leigh stated, "to get off a train, bus, etc." is more informal sounding than "to alight from a train, etc." In American English, "to get off a bus or a train" is the most common and most natural way of stating it and actually, people here even use it in a more formal setting too such as speaking with their boss.

Of course, in music, one might choose one word over another for various reasons. For instance, to sound more informal, to sound more innocent, to sound more wholesome, to sound dirtier or more offensive as in some rap music, etc. That is, you will most likely never hear the word "to alight from a train" in an American bluegrass or country song. It is too "sophisticated" to use with bluegrass and country music lyrics and actually hearing that word sung in either genres would sound really weird even if it is a synonym in my opinion. It would be like mixing "rich words" with words for "common people". Hehehe. Instead, a bluegrass or country singer would use "to get off a train or bus". That's exactly what Johnny Cash does in his song "Let There Be Country" when he sings "And somewhere downtown Nashville a young man just got off the bus with a guitar in his hands a pocket full of songs." The same goes for rap songs which are intended to sound "thuggish". You won't hear "rich words" in them either. :)

Who can betray a secret?

Tell me please why do people often use phrasal verbs instead of their synonyms?

Phrasal verbs are commonly used in informal writing and speech.

Example:  Formal "To postpone a meeting", Informal "To put off a meeting"

                        "To alight from the train",             "To get off the train."