if only online services could implement an identity check that way.

Outside of banking and taxes, I can't think of a single online service that I want verifying my identity. Ostensibly, these services just want to verify if you're of legal age. The small, binary question of "Are you over 18?" should require much less private data than would be needed to verify my identity.

They can! In fact, in the UK some government services do this already however there's a couple of drawbacks:

1. They'd need to use some kind of companion app for each device since the web APIs won't give them access to this.

2. For us at least the NFC chip still contains a digital copy of the photo so it's not really any better since I don't know what data is going to be sent by the app.

My point wasn't one of convenience, but more one of privacy. What's the difference between someone demanding a photo of your ID (and then storing it) and someone taking the same information from an NFC scan and then storing it?