No other dolls offer such potential for realism as baby dolls. For one thing, no other dolls are so similar in size to the real people they depict.
A baby doll is almost the same size as a baby while even a large 18" play doll is on a much smaller scale than an actual child. Details on the face, body, and clothing of baby dolls are much truer to life than other dolls, which must inevitably stylize and simplify some details.
Indeed, even sometimes the line between baby clothes and baby doll clothes can get blurry, or disappear altogether.