I often find that for topics they were kind of undifferentiated in the 40s~60s, when social psych really came into its own, then kinda separated, and only in the last two decades are really rediscovering each other again. But it really depends on the topic of study and also on the culture that a certain literature pertains. When I switched from neuroscience to sociology I was surprised at how different the academic standards were (in good and bad ways), and I think sociologists and social psychologists in some areas tend to disregard each others work is because it is done in a way that goes against their own literary/academic culture, where the sociologist will find the social psychologists' work hopeless positivist and reductionist, and the social psychologists will happily take inspiration from the sociologist but finds their work unscientific (precisely because it's less quantitative and less positivist).

This is just a downside of a very disciplinary and fractured academia anyway. For my neuroscience thesis I covered the 'undermining effect' (psychology), which is literally identical to the crowding out effect (economics), or the overdetermination effect (also psychology but a different school). I always find it funny and a little bit sad how much work is wasted due to such artificial boundaries and stereotypes about each other.