The background layer is locked because it is the background and, by definition, can't be moved or resized without changing the dimensions of the document itself. In a single-layer image, like a jpeg, that's all there is. If you think about it, it's clear that you can't move the background around or resize it, because that is the image. So it isn't that Photoshop "puts the image on the background layer." The background layer is the image.
A non-background layer has special properties that a background can't have. The Photoshop engineers call it "big data" because it can be far larger than the visible image area. A layer can be moved, scaled, and otherwise manipulated without changing the dimensions of the image as a whole.
If you run a filter like Lens Correction or Liquify on a background, it will be promoted to Layer status automatically so that the filter can run. When you promote the background to a layer yourself, you also open up all the possibilities, as Jin pointed out, of layer effects and other manipulations that can't be applied to a background.