Because every human has cheekbones. It means that, under sufficient leanness and/or the right amount of hard drugs, anyone can have "hollow cheeks". It's just the matter of lighting, and how pronounced they will be, which is a function of cheekfat and the zygomatic arch.
This is the face of an Auschwitzmaxxed drug addict. Visible sunken cheeks and lower third with defined jaw definition.
View: https://www.instagram.com/reel/CyRh854LA_7
This is him drug-free. A boneless subhuman.
The same reason why anything else is universally viewed as attractive. Hollow cheeks are indicative of healthy craniofacial development and good genetics, both of which are clearly desirable qualities in a partner. Genuine hollow cheeks come paired with good fat distribution, projected zygos, and wide gonions. It's how hollow cheeks are formed.
It's also why buccal fat removal often looks uncanny because it's the bones that matter, not necessarily the hollowness itself. Having hollow cheeks while being boneless just makes you look skeletal and malnourished, which is better than being hig body fat, but it's still not to be confused with prominent cheekbones produced by good genetics.