Because the issue is the culture, and the culture is entirely in the responsibility of middle management.

If an IC behaved like this then it's would've been the responsibility of the middle management to let them go when it started. So it'd still be on them.

And that's ignoring that issues like this have historically always started in middle management.

Also I suspect you're looking at it from an individual level: one middle manager on their own obviously cannot have enough impact to change this culture, so it's not the "fault" of any one manager. And that's the reason why the heavy handed approach is necessary, because the bad culture has settled. Anything any one manager may try to improve their ICs work life will inevitably get soured by the next level.