I sort of understand why, because I work with people who specialise in this stuff. And in fact it happens illegally in parts of the world: people clone SIM cards so they have the same IMSI and then rely on being in sufficiently different parts of the network that there is no conflict between masts. But to do it legally would require different international standards to those that define how LTE etc work. In essence, though, you are talking about the network holding different phone number- imsi mappings, rather than necessarily any change in the phone. As I understand it - which isn't that much! - doing this using voice over IP works because every device has a separate IP anyway and that is the primary routing mechanism.