This is overwhelmingly due to the fact that SharePoint sucks.
Not having full CRUD capability in the UI of the comments is an example of what happens when you run a product development project using Agile / Scrum. Someone put a user story in the backlog saying "Ability to add comments". But no-one parsed the user story against the templates for established practices, like: permissions, CRUD, governance, to fill out the functionality to a rounded, holistically complete solution.
This kind of half-baked cludge is all over the SP product. It *is* the SP product. As a colleague once said on seeing SP 2010: "SP is like the proverbial you cannot polish - but you can roll in Agile / Scrum glitter."