1. We don't (yet) inherit antibodies to SARS-CoV-2 from our mothers.
2. SARS-CoV-2 has not been forced to make evolutionary sacrifices to evolve to escape immunity.
The virus has the freedom to become optimally bad and our immune systems were optimally naive.
Over time the warfare between our immune system and the virus should cause SARS-CoV-2 to look a lot more like HCoV-NL63.
That's a sufficient explanation of the differences in disease severity without having to troll through medical articles.