Because then the reviews for the connected PRs ends up being hideous to track.
Either you can:
• Submit the PRs one at a time... which is awful. Why not work on them in parallel?
• Submit your series of PRs all at once, then you have to set the PR base of each one to the following one to make the github diff view actually reasonable for reviews. THEN, when you have a code change based on feedback on the first PR, you have to merge it into all the later dependent PRs. THEN, you have to have to jiggle the PR bases very carefully when you're landing the PRs, otherwise it will merge them all into each other, and you end up with one ginormous commit landing—defeating the point of the small commits. AND unless you're using a squash based workflow, you'll end up with that giant merge mess in the middle of your commit history.
Either you can:
• Submit the PRs one at a time... which is awful. Why not work on them in parallel?
• Submit your series of PRs all at once, then you have to set the PR base of each one to the following one to make the github diff view actually reasonable for reviews. THEN, when you have a code change based on feedback on the first PR, you have to merge it into all the later dependent PRs. THEN, you have to have to jiggle the PR bases very carefully when you're landing the PRs, otherwise it will merge them all into each other, and you end up with one ginormous commit landing—defeating the point of the small commits. AND unless you're using a squash based workflow, you'll end up with that giant merge mess in the middle of your commit history.