We in this forum did not design or write the code for Lightroom, so we cannot answer questions about "Why"
You can add a copyright to your photos using the Metadata Panel in Lightroom. This metadata will appear in the export, if you so choose.
My concerns are caused by another question: What is a recommended workflow to add e. g. a xmp formatted copyright statement (created by Bridge) to an imgage file? Should I add the copyright xmp file to the raw file or to exported JPEGS?
Try this. In Bridge, apply the copyright statement to an image and then import that image into Lightroom, ensuring no metadata template is shown in the Import dialog. Then save a new Metadata Preset so you can apply it to other originals.
If the image is already in Lr, save metadata back to the file, then go to Bridge and apply the copyright template, and then use Metadata > Read Metadata. You can then create the Metadata Preset.
I'd suggest a couple of other things.
First, check out the excellent plugins by Misters Beardsworth, Ellis, and Friedl. All have plugins that allow much more control over metadata in sidecars or anywhere else, and make Lr a lot easier to use.
Second, try Bridge. If you look at a folder of images, and turn on the option to "show hidden files," you will see all the sidecars. Then just click one and open in your favorite text editor if you wanna edit it directly. Then back in Lr you'd have to read from that sidecar or synchronize to get the data into Lr itself.
It's a lot easier to add something simple like copyright within Lr, but this can be handy for debugging issues with metadata. Also, in some respects Bridge handles metadata better than Lr, and itself can be a better choice at times.
Thanks a lot for all contributions to this thread!