For the record, many brands do hollow out their gold bracelets and cases...
Rolex used to do this for years until right around 20 years ago they went to a more solid construction. Many brands, including top tier brands, still do hollow construction even today.
But, yes, Bulgari is a little unusual that they illustrate this so clearly, whereas other brands don't make it visible at all.
The reason why Bulgari does this is for weight savings and weight balance. The case would feel a little light in the middle if the bracelet were solid. And clients are generally accustomed with the cases being the heaviest part. But because the case is quite thin and the movement fills up the case so thoroughly, the balance would be shifted and instead of a "top heavy" (top refers to the case) it'd actually be a "top light" watch which is unusual in the watch industry.
So it was done for balance. But I agree with you! I find it very strange to see! I understand why it's done this way. But I still find it a little strange!