Because Slack's product assumes you have an HR department, and that's an HR issue.

True story: at a former employer, a colleague made an HR complaint that in reality was well founded due to abuse. At one point in the investigation, HR told the two people not to interact with each other directly, but for unavoidable work reasons they were still in the same team-related Slack channel.

A mute or ignore feature would have been great to lower the victim's anxiety level during that time due to HR's unhelpful handling of the situation. The victim's finances and general life circumstances did not make "just quit" a good resolution.

(Oh, and the end result: HR fired the victim, with some severance paid, and kept the abuser a while longer. The abuser eventually got fired but probably not due to this incident.)