Because with unit testing, I miss and can miss so much conditions. If I have high level error, sure, I will figure it out - but it is part of development process rather than something breaks live and then you have to figure that out anyway.
Ofcourse Unit tests have their place where I want to test that this input produces particular output (for example some parser, sanitizer class etc.) and I want to receive a signal when my future development breaks some stuff.
Tldr: I don't think that having a very specific "what went wrong" is that important. I'm grateful when tests fail, because that saves me from mistakes going into production. It's like a safety net.
Unit tests are also important, as you said.