> Why can't it be in a team slack?

Only a few people can adequately explain themselves through slack.

It doesn't help that a lot of managers are _bad_ managers, and don't/can't/don't know how to run a tight 1:1.

the point of the 1:1 is to provide a high bandwidth way of getting worries and steers from employees to management and direction back to employees. if there is nothing to talk about then cut the meeting short.