One explanation could be that if you are a hammer everything looks like nail. One problem with web dev in general is that we have multiple backend technologies

One explanation could be that if you are a hammer everything looks like nail. One problem with web dev in general is that we have multiple backend technologies, thus creating a complete client and server rendering framework requires you to integrate it against multiple backend technologies. But if you assume that the best API is JSON over HTTP then you can completely ignore what makes sense from the perspective of the chosen backend technology.