And add that you have dozens of such patches that you need people to review, and their reviews have to be distributed to a committee of senior developers once they're received, and so on and so on...
A journal may have dozens of submissions at various stages of the peer review process. Each requires two or three peer reviewers, who are busy academics who will often fail to meet the deadline. The editor has to track all of these, follow up with lagging reviewers, and then put together all the reviews for a paper to make the final decision.
Doing all that by email would be a mess. Reviews would get lost, the editor would lose track of which reviewers were sent which papers, and so on.
So the online systems make sense. But they all suck. I've reviewed for several journals which use the ScholarOne system, for example, and despite all being hosted centrally, they inexplicably require separate logins. The website is a clunky piece of crap. If they just had an automated system email me a PDF and ask me to email back a review, and had their script grab reviews from the inbox and format them for the editor to review, it'd be a lot easier. Or if they emailed me a personal link that let me view the paper and leave a review, no login required. But alas, they must overcomplicate it...