I think there’s two separate things going on here.

One is that software started being designed in a one-size-fits-all manner for a “uniconsumer” that vaguely represents every user the software will ever have. Personally I think that analytics-driven development is largely to blame here; through those you don’t see user groups, just numbers going in whatever the product manager deems to be positive and negative directions. We don’t design for specific users, but rather to produce desired metrics.

Second is that professional UI design got flooded with designers from other disciplines (e.g. print design) who only really understand the visual half of software design. The functional, interaction, and user research parts completely fell by the wayside as the job became more focused on coming up with designs that would do numbers on Dribbble and look pretty in a product page or presentation and is driven mostly by vibes now.