Email was always designed for human to human messages. Patch files need strict formatting (like anything that needs to be machine readable). That means you need to install extra tools, point them at your mail server, configure the mailing list address and proper formatting for each project, then find the right commit range and type in the description into a command line editor every time you send a patch series. The alternative is doing everything manually and that takes even more effort.

Compare that with GitHub where the workflow is push changes to fork, select branch, click to create a PR, write the description in a convenient web form and submit. Less friction and less opportunities to mess up. The solution to a centralized forge is a decentralzied forge, not no forge.