In my experience, which may not be common, UI people tend to be more social than backend developers. In reality, and this is not a judgment as to what is right and wrong, but who is more readily able to push for change? Someone that is deep into the software, or someone who is closer to management and able to communicate more effectively (as far as socializing and building a rapport?). Again, I'm not saying it is right, I'm just going by the reality of how software works. This is a good reason for independent software developers to exist! I agree that there is huge business value in writing correct software that is performant, and gets the job done properly. I agree with you that Slack's UI could be improved drastically. The problem is that it's driven by money rather than productivity. I'm pretty sure you and I both agree on how software SHOULD be, but that is not how software actually is, and I don't think it will be until it gets out of the grips of large software developers, or we start to write for performance in an attempt to make software that is environmentally performant.
I'm not sure if we ever get to the point where developers need to start worrying about performance for environmental purposes, that we'll survive if the performance impacts have been measured properly. I'm not trying to be depressing, but I just don't think that it would ever happen until it was too late. Maybe there would be a minority left of society to do this, but that would be building back society. Doomsday scenario. I don't think "we" are smart enough to deal with any catastrophic situation until it's too late.