a) Sign up for an account in your forge: Do contributors really want another account? Does your captcha/email verification actually work (I've encountered ones that don't)? There are also forges that require you to ask for an account which is another hurdle.
b) Send an email: Configuring `git send-email` is alien to many contributors and may not even be doable in some corporate environments (OAuth2 with no app passwords allowed). Diverging from this is error-prone and against social norms which the contributor may not even be aware of (until they get flamed in the mailing list). You are also giving up automated CI which is a big part of the contributor feedback loop.
To be clear, going independent does indeed work for small personal projects (do not care much about contributions) as well as established ones (large incentive for new contributors to jump over hoops), and I'm fully aware that a lot of HNers do not see the need for those "niceties" provided by GitHub. But I feel that people often underestimate the barriers that they are putting up.