There’s much less point to having them on an iMac because there’s no space constraint.My speculation would be that ears are going to be a "pro" distinguishing feature or we'd have seen them on the 24" iMac.
A laptop has to fit into a bag, so you care about how large it is and want to squeeze as much as possible into that space. A desktop, especially an all in one, can afford the odd extra cm here or there without any inconvenience.