Given that we vote for entire comments, we don't expect people to split their comment into many sub-comments.

At a wild guess, that you may upvote a comment that you support – but there's a second part that you wouldn't agree with. Or that people should split up their comments so that people can upvote/downvote individual parts?

I think OP is referring to comment where multiple talking points are made, but you can’t selectively apply your upvote/downvote to specific parts of the comment. You either upvote it all, or downvote it all.

So if I start talking about how pineapple is ok on a pizza, a downvote for this take also means you downvote how I’m trying to clarify OP’s message.

I hope I made it clearer by editing the post. Let me know if it's still not clear :)

You would reply twice to the same comment, generally

Well yeah it's not like

My post tried to convey that most people do this:

[Original post by OP about COVID-19] Comments:

• User A: "COVID-19's symptoms can vary from person to person, and the vast majority of people do not present life-threatening symptoms. This can make it easy to conclude that COVID-19 cannot possibly kill someone [edit: here's a source that shows that many people actually believe COVID-19 cannot possibly kill someone: statistics.net]. This is an unfortunate situation, because trusting the science can lead people to use appropriate masks and reduce its spread. [edit: added the word "appropriate" thanks to User C]
  • User B: "Really? I don't know anyone who believes COVID-19 cannot possibly kill someone"
    • User A: "I responded to you by adding a source to my original comment through an edit"
  • User C: "My niece used a cloth mask in the Prague metro and still got COVID-19. I suppose the type of mask matters."
    • "You're right! I'll edit my original comment to reflect that."
  • User D: "I'm sure you won't reply to this comment if I say that I don't accept science."

and they don't generally do this:

[Original post by OP about COVID-19] Comments:

• User A: "COVID-19's symptoms can vary from person to person."
• User A: "Many people think COVID-19 cannot possibly kill someone."
  • User A: "My source for this is statistics.net"
  • User B: "Really? I don't know anyone who believes COVID-19 cannot possibly kill someone"
    • User A: "I responded to you by adding a source to my original comment through another comment"
• User A: "Mask usage helps reduce the spread of COVID-19."
  • User C: "My niece used a cloth mask in the Prague metro and still got COVID-19. I suppose the type of mask matters."

The point is that we usually don't split our points into many comments of the same level. Levels here refer to this:

• Level 1 of a comment tree
  • Level 2 of a comment tree
  • Level 2 of a comment tree
    • Level 3 of a comment tree
• Level 1 of a comment tree

When I say that we take that for granted, I mean that I don't see people splitting up their comments in the same level. Neither do I see people talking about splitting up their comments. In other words, neither in practice nor in discourse do people split up their comments.

Edit: Rewrite for clarity

Interesting. What makes you say that?