This is a fair question that is worth discussing. The short answer, is because that generally requires money and resources long-term that are not already available or allocated during the course of the dig.

Covering exposed features is the only way to “protect” them from the elements, and from the public. Furthermore, it also leaves open the possibility of uncovering them in the future for additional research or examination. This is actually a common practice in archaeology, much more than people realize.

Which bring us to the fact that the purpose of archaeology as a science, is not to protect every uncovered feature or even every discovered artefact, but to use these materials and their placement in situ to gain knowledge and insight into the human past. As such, the material objects are often of little value unless entirely unique, no museum or archive has endless storage for every object recovered. In fact, artefacts discovered on digs that cannot be added to some collection and are of a known factor, are usually discarded en masse and reburied.

It’s possible that what you’re suggesting could happen in the future, but that would require planning, funding, and time for it to happen. Without covering up the site now to protect it the way it has been found, there wouldn’t be time for any future planning or funding to even allow that decision.