Cause Samsung is allowed to be sold in more countries compared to Xiaomi
Even if all of the three were global, which BTW the 15 Ultra is available globally sans the US, the software of the Oppo, Vivo and Xiaomi remain barriers to entry with them ripping off everything from iOS, paired with their über-agressive battery management watchdogs.
Yeah that works in China where people are accustomed to iOS, but in the West, people will see it as a cheap ripoff.
Hence why Samsung still sells like hot cakes despite the cameras not being on the same level, or the lack of bigger Si/C batteries and the like.