Earlier quoted context omitted.
Right, it's "I have to send 5 PRs to 5 different repos, get them all cross merged, and then at the end it's wrong anyways so I have to start all over". Multirepo management is extremely frustrating compared to "it's all in the same folder".
The process you're describing looks like some trial and error PRs... Multirepo also allows you to roll out that change incrementally instead of big banging all the time.
Meanwhile reviewers don't have context about changes, so it's easier to get lost in the weeds.
It's not always this, of course. But I think that way too many tools are based on "repo" being the largest element, so things like cross-repo review are just miserable.