The reason was probably that no one ever needs to delete just a subset of cookies from a site. I certainly never have, and I'm a pretty technically-minded user. (Other than for development, in which case I'm using dev tools anyway.)
However, needing to delete those cookies without first viewing the page due to side effects - that is a very niche use case.
Manually interrupting most flows is practically impossible. Too fast, and can chain across a half dozen (sub)domains near instantly. The web hasn't been that slow or simple in a long time.
Yes, as mentioned, from atrocious engineering.
See also that Windows can break the desktop due to their own advert.