Several reasons I can think of:

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).