Because the job itself has now changed, and they haven't. Their output speed might have been eclipsed by that of the engineers who efficiently adopted the new tooling.
Where I work, the power dynamics have shifted wildly. There are a number of senior engineers who refuse to touch the stuff, and as a result, they can barely keep up with their peers. Some of our juniors are now running laps around them.
When a stranger to your craft can now teach themselves what you know, how to do your job, and even how to automate your tasks in the span of the same workday as you, all while reliably being able to gauge the innacuracy of the output they're reading, how much longer do you really hold relevance?
How do you mean "barely keep up" and "running laps around them" ?
Are the juniors increasing economic productivity or just pushing lines of code?
</retired from being measured against a random number generator>
They aren't teaching themselves anything though are they, they are basically getting AI to do their work for them.