I don't think it's that we're supposed to be bad at things, it's that we're expected to already be good at things. This still applies (irrationally) even when:
1. There is no mechanism provided for becoming good at the things we're expected to be good at.
2. Most people aren't good at the things we're expected to be good at.
The underlying thing here, is that ignorance, a temporary condition, is conflated with stupidity, a permanent condition.
Carol Dweck puts this another way: she describes a "fixed" mindset, where people believe that mistakes are examples of their inherent value, and therefore fear taking risks, and try to hide their mistakes. This is contrasted with a "growth" mindset, in which mistakes are seen as a necessary part of growth and risks are seen as learning opportunities.
Also related is Brene Brown's theory of vulnerability as being a necessary part of forming relationships.
I will say, I'm not entirely convinced by all the details of either Carol Dweck's mindset theory or Brene Brown's vulnerability theory, but the generalities seem to be evident in my own life.
EDIT: I'll also add that this conflation is particularly damaging to people and communities where intelligence is highly valued, and people base much of their self-worth in being knowledgeable. My personal experience is that growing up I based a lot of my self-worth in "being smart" so anyone disagreeing with me or pointing out something I didn't know felt like an attack on the very thing that made me feel worthwhile. Building real self-esteem made it possible for me to start seeing that when I don't know something that doesn't decrease my value as a person. This has allowed me to admit I don't know things more often, and actually fill in some of those gaps with real knowledge. I'm still a growing person, and I'm still not good at this, but the progress I have made has improved my life immensely.