Some of my open source work is done on mailing lists through e-mail
It's more work and slower. I'm convinced half of the reason they keep it that way is because the barrier to entry is higher and it scares contributors away.
You mean, assuming everyone in the conversation is using different email providers. (ie. Not the company wide one, and not gmail... I think that covers 90% of all email accounts in the company...)
Email at a company is very not decentralized. Most use Microsoft 365, also hosted in azure, i.e. the same cloud as github is trying to host its stuff in.
365 is not hosted in Azure. Some of the admin portals and workflows are, but the normal-employee-facing applications and APIs have their own datacenters.
It's more work and slower. I'm convinced half of the reason they keep it that way is because the barrier to entry is higher and it scares contributors away.