I think the issue is that while the phone network standards allow a single sim card to be authorised to connect and roam between different phone networks, it relies on the networks themselves having infrastructure in place to, for example, migrate IP addresses from one provider to another in realtime.

Phone networks didn't want to make the massive infrastructure changes to allow that, so Google took a different route. Instead, you effectively have two services - one from each network. I believe they both have their own phone numbers etc. An app switches from one to the other, and all calls and data are directed over VoIP and VPN to googles servers, which then route calls and data packets so that a switch can occur mid-call without the call being dropped.

All that added complexity on-top requires a fragile software stack on the phone, which is why Android phone support is patchy, and iOS support is only partial.