Seems to me that it's a practical form factor in response to prevailing market reality. Last I checked, every single bit of market research indicated that a super majority of phone owners use cases of one variety or another. Not all of them for sure, and I ran without for a long time myself, but the overwhelming majority. So if most are going to be adding some level of thickness and mass to their devices regardless, and that will necessarily have a hole for the camera system which would just be empty wasted volume otherwise, it makes sense to strike a compromise and try to make more use of that without making it completely unusable without a case.

But cases are the major form of physical customization most people have. Cases can provide physical keyboards, battery, environmental protection, texture, and aesthetics in a variety and individual granularity that would be impossible for any OEM to handle themselves. Fundamental to cases though is that they can only ever add, never subtract. So it seems very practical to me to design a device that is as thin and light as possible within a floor level of raw performance, since that represents the base state. Then individuals who want more battery life, more extreme environmental usage and so on can add from there, but those who don't aren't saddled with it. Cameras feed into that, if anything for many heavier duty case users it'd be more optimal if they were any even bigger bump. They represent a compromise.