Because google-fi isn't a real network. It's not some spec that you can implement. It's done by play services blobs. Even Android cannot handle it. Only play services can.

iOS apps cant make iOS understand it's allowed to switch between Sprint and T-Mobile - Fi is an MVNO and rents access to their networks. Fi in iOS is locked to T-Mobile.

Isn't that the job of the SIM card, not an app? SIM cards have been able to support roaming and multiple networks since day 1.

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.

Couldn’t you use a dual sim though, or several virtual sims and depend on the phone to switch?

That's what they do.

But while switching networks, your phone number will change, your IP address will change, etc.

Google's layer on top hides all that.

My impression is that it’s handled by proprietary software written by Google itself.