Because of “polished”. The rims are"polished",ie bare metal with a lacquercoating. The lacquer coating always fails. These are, ultimately, cheap wheels. BBS wheels aren’t lacquered, but use a clear anodized layer, hence are very expensive.
The alternatives from “Japan Racing”, Dare, Lenso, Calibre seem to be copies of/inspired by the BBS Super RS split rim, only these are fake split rims, with split rim “bolts” that are plastic and fall off.
Another to keep an eye on is Maxilite. Anothe Swiss-German company that actually makes wheels in China. They specialise in replicas of classic designs. They brought out a 15x7 BBS copy for vintage BMWs that has a rim style really close to the BBS lip.
Looks like they are going for all silver paint finish, not bling polished rim. Sensible. But these only came out in July, with a BMW 1502 fitment only so far, which means the offset is out of whack
The 1502 used a 4x100 stud pattern so these will fit, but with a ET10 offset, will stick out too much. Might be worth enquiring with Maxilite at https://www.maxilite.ch/en/about-gtc to see what is in their pipeline. If they get the fitments right, these I expect will be a hot seller. Later BMW 4-stud, VW Golf 4-stud and MX5 NA are all pretty close (tough MX5 will ned spigot rings).
I had brought previously their Minilite style wheels, and found them very communicative. And they seem to have revised their wheel design in response to my comments (though, for these wheels, they used polished rims, which, sigh, after 3 years, started corroding).
Comparison with genuine Mazda-BBS. Its very close.