Believe it or not, the problem is likely that they are consulting UI "experts". Many designers responsible for this stuff (at least the ones I've worked with, and I can extrapolate based on what other companies have released) haven't been in the industry for very long and they don't really have the practical experience in using computers and applications to recognize the harm they are inflicting. Combine this with the fact that the one of the main ways for designers to get pay increases is to pad their portfolio with dribbble-worthy form-over-function designs and you start to get things like the OP is lamenting. And the Slack redesign.