Maybe it’s why you aren’t a product designer at Apple? No reason why it couldn’t be integrated into the bottom/base with a moulding to exit the cable. They could also ship a power adapter/cable that effectively is usb-c to PoE and not need any other port. It’s not rocket science it’s a cable.
it's a thing that takes space. the overwhelming majority of people who purchase a homepod mini are not going to use PoE functionality, so those engineers would need to redesign the internals to make space for an RJ45, that's a decent chunk of space in a product that isn't very big.