It’s not that they didn’t know that they could. It’s that they didn’t think they would.

Because—and I say this as a user of Duolingo who first started using it after the old comments were made read-only, but before they were removed entirely—it’s fucking insane that they did. Those comments were so useful to the user. I don’t know how many times I went to them to have some aspect of the lesson explained to me because the app itself doesn’t actually do any real “teaching”, it just tells you that you got it wrong and what the right answer is. The comments from users helped explain the nuance in word meaning, or the relevant grammar rule, helping add enormous value. By removing them they are literally making their product worse for no gain.

People thinking that they’d act rationally wouldn’t expect that.