Why are the final composite images of the Milky Way inherently historical?

They are a collage of light that took different amounts of time to reach us.

The beautiful, finalized images presented as visualizations of the Milky Way are mosaics assembled from disparate data points collected across the entire electromagnetic spectrum—radio, infrared, and visible light. Because the galaxy spans roughly 100,000 light-years across, the light forming different parts of this composite image traveled for vastly different durations to reach Earth. For example, light from the near side might be only a few hundred years old, while light from the far edge of the core region might be tens of thousands of years old. Consequently, the final visualization does not represent the galaxy as it exists in a single, frozen moment, but rather acts as a collage representing the galaxy as it was at various points in its past history.