To assume that it is a fabrication, is an option, however, it is not the only or best option, given the contemporary witnesses to substantiate these claims, given the acceptance of Luke's gospel by those apostles who were there at that time, given the values and ethics of the followers of Jesus.
Instead, let's look at some others much more plausible reasons:
- Luke was concerned primarily with the humanity of the God-man, Jesus. He has much detail with Jesus' care, His touch, etc. As such, the addition may have been an important detail for Luke to add, given what he was trying to accomplish with his writing.
- Maybe he added it because it appealed to his line of work, and was fascinating to him. That is a valid option.
- "Many have undertaken to draw up an account of the things that have been fulfilled among us, just as they were handed down to us by those who fro the first were eyewitnesses and servants of the word. With this in mind, since I myself have carefully investigated everything from the beginning, I too decided to write an orderly account for you, most excellent Theophilus, so that you may know the certainty of the things you have been taught." -Luke 1:1-4. The reason for this letter, is to give a very accurate account of what Jesus said and did.
I feel the most comfortable saying #1,3 are the most plausible reasons why he includes this story. Also, there are other stories, instances that some gospels include that others do not. Because another excludes it doesn't mean that it is false.
For example, Often times, when I do marriage counseling, one person will tell a story one way, and another will add more details. Sometimes they will also give a different bent to the same story. Does that mean that one is lying, or could it mean they are less detailed? Then, if the stories don't align, I will pursue asking a third-party, an eyewitness who can substantiate the claims (much like the contemporaries of Luke who were eyewitnesses to all Jesus said and did).