I don’t know why you’re getting such combative and inaccurate answers, but this is an excellent question! It’s called backfilling, and it’s an extremely common practice at archaeological digs all over the world for a number of reasons.
You can’t beat the natural processes of the earth for preserving much of what is found. It must have done a good-enough job up to the point of excavation, otherwise we wouldn’t have found whatever it is we found. So it is usually more efficient, cost-effective, and functional to backfill an area that you know you’ll need to come back to later.
Excavation is inherently destructive, you can’t “repeat” the process like you can with hard science experiments, so archaeologists are encouraged and often required to preserve (meaning not dig) areas of a site for future research when we know our technologies will be improved. And if you can’t dig a whole feature properly in one season, backfilling it to preserve your progress until the next dig season is incredibly common practice.
The natural processes of sedimentation do a much better job at preserving something that has already been exposed to the elements than most of our modern techniques. So if there is an important find, it’s often easier to backfill it with clean sediment to ensure it’s still well-preserved when the researchers are ready to properly study it. Often a layer of geo-frabric is laid down under the backfilled material to mark where the area of interest starts, and so that you know you can dig quickly without worrying until you reach the fabric.
It’s harder for looters to know where to look for “treasure” when a site has been backfilled between seasons. It’s often one of the only security measures in place at sites that are under excavation year after year.
There are many other reasons for why backfilling is a pretty standard procedure at archaeological sites. I don’t know specifically if backfilling is common at Pompeii, but I’d imagine they must do it every so often. Pompeii is one of the most famous tourist sites in the world though, so it’s probably not the most representative example.
Regardless, your instinct is right, backfilling is incredibly common, and often the best way to preserve a site for the future. Don’t let the haters get you down!