You may not have been talking about culture, but I believe you missed that culture is a big element in why homogeneity in design is currently being critiqued.
There are two different versions of ‘adding variety’ in play here — Design as visual language, and design as content representation/strategy. In terms of visual language, you are entirely correct. Writers do not reinvent language every time they write a book; so too should designers use common structures, tools, themes, imagery to make products accessible to users.
Design goes deeper than looks, though. Design is ultimately about rendering intent. If designers don’t have a wide enough understanding of all the stakeholders involved — their needs, their experiences, their values — the designer risks not just making safe, boring designs, but dangerously inconsiderate designs as well. A job application format that is impossible for someone with a visual impairment to use, someone on a public library computer, or someone who speaks English as a second language, for example. The more limited your range of experiences, the more likely you don’t even know that the many ways the thing you’re making is excluding people.