Because those changes might depend on those other changes. Git merges aren't linear. They're branched. And PR reviews are meant to just examine the merge of 1 branch back into master. They're not really meant to review multiple steps along the same branch to make it easier to review.

There are so many times where I want to create 3-4 Merge requests that all build on each other along the same branch instead of creating one giant MR but the UI for reviewing them doesn't really work that way.