Hi FRiend!! great question... I had to look this up too cuz I'm not actually too sure but my immediate response & the best way I can describe is you don't actually call ur partner "baby" as in a literal baby right??? but its still an affectionate term! same thing!!! โค๏ธ
From what I learned, it started off as a familial thing and an age based descriptor (something you call an older male person) BUT because traditionally older brothers are often seen as protective, caring, strong, or someone who you can rely on -- if you date a man older than you it kind of fits the same social role....? (hate to prescribe to gender roles just trying to explain the origins)
That's how it changed as a term but in general, it just is a type of nickname you call your partner just like nuna is older sister but also a nickname! Or baby is a BABY but still a nickname! Hope that makes sense~~~ | 
