1. Google and Apple have a much larger ecosystem and are entrenched in their OSes, which means that they have a much better picture of the user than any government app ever will. They also have surveillance mechanisms that government apps are unable or unwilling to implement. This helps detect and prevent fraud (fraud prevention is mostly just mass surveillance used for good).
2. The eIDAS standards enable anonymous assertions about your identity. This lets you prove your age to a website / app without revealing any other information. There needs to be a way to prevent you from generating millions of such assertions using one ID and giving them out online to anybody who wants them, verified or not. The way you do that is by limiting their generation to trusted hardware, using hardware attestation mechanisms. Google and Apple provide those.
3. Pure laziness. It's an issue that <1% of the population cares about (which is hard to notice if you're in the HN bubble). Almost nobody uses a modern, eIDAS capable smartphone without a Google or Apple account. They may have decided that the part of the population who cares about this just isn't worth pandering to (just like some government institutions may decide that vegans aren't a part of the population they're interested in pandering to).
There can be good reasons for a bad thing, and it's important to factor them in when having a discussion.
Anonymity isn’t anonymity if you can’t generate millions of them cheaply.
Either the government secures internet payments themselves, which means spending now to do so, coming up with a plan, ... or they can have Apple/Google do it.
You can smell where this is going, no? This is how the EU is looking to make any kind of internet authentication go through them. By providing companies like telcos with an online identity that says "if a customer clicks 'buy' logged in through eIDAS and they don't pay, EU courts will if needed get the money from their homes, their mothers, sell their dog to make sure you get paid".
Then things like forcing kids off the internet, the always returning porn and copyright regulations rules and so on will follow.