My best guess would be that it's a placebo to keep toxic commenters from escalating a situation. They probably looked at a few case studies and a lot of statistics and found that people willing to engage in harassing other users were less likely to engage in acts like leaving nasty messages on a user's videos or other toxic activity if they had the agency to simply dislike the comment that made them upset and move on. Removing the placebo gives people like this few desirable options to let off their aggression, and they're probably more likely to engage in arguments or otherwise make people's days worse.

YouTube's comment section has been infamously terrible since the beginning, I believe YouTube and Google both have a vested interest in trying to improve the quality of it to make their service more marketable. It's also worth noting that the system was originally instated as a crowd sourced way to deter spam, and now that YT is more reliant in algorithms and such to prevent that, it selves a less critical purpose.