Engineering issues aside, I suspect the chief reason we don't see this is also the same reason we haven't seen expandable storage or swappable batteries on the iPhone. Not because Apple wants to gouge their customers, but because this would undercut their design philosophy.
Apple is all about minimalism and purity in hardware design. I suspect Apple decided long ago that having their product accommodate a removable battery would just sacrifice the integrity and beauty of the device, and they are probably right.
With regards to the iPhone, having a solid frame with an internal battery would make the phone more durable and the battery would last longer too, since it can be built larger within the phone. Sure, Apple could just build the phone larger, but doing so would compromise their design principles.
Same thing here with the AirPods. A tip that can be unscrewed is likely an additional point of weakness which you won't have if it were just a single sculpted piece of plastic. Making the AirPods thin, light and uncompromisingly simple is the goal here, and Apple is not going to compromise that just for a feature only a small subset of their user base is ever going to take advantage of.
Where Apple is concerned, they have never been about making products with the most features, but about cutting down a product to their most basic form, with nothing standing in between it and the user. People always try to make it about Apple grabbing more money from their users wherever they can; I will argue that this really isn't the case, but money is always a bonus).