Stupid question from a noob: why not just use some raspberry pi zero, a camera sensor and some ONVIF server? That's what most of those cheap chinese IP camera are: a linux board, a camera sensor and some compression chip.
Because the camera comes in a neat box with resonable weather tightness and it is easier to setup and connect. IP cameras also have the advantage of not needing a power brick.
Your other points withstanding but there are some slick PoE boards available for the RP0s.