[idle_zealot]

It's not an either-or thing. Every case of WebUSB flashing I've seen also offers an open source CLI tool you can pull down and use instead.

[smrq]

Perhaps because WebUSB is not well supported? Just a hypothesis, but pushing it as a standard might encourage it to become an either-or thing.

[wayvey]

Many of the WebUSB flashing tools work fine offline, so one can just save the web page locally and use it that way.