- they provide a service, not software

- you'd also need a maintained /24 of IPs with good email reputation

- unless you want to only use an email client, you need to do lots of testing on the web editor part for how the formatting gets interpreted in other clients, how to render weird emails correctly, how to protect the privacy in external resources, and lots of other things

Even with an LLM that's both a multi-week implementation and an infra management project for life. Their users in general can't do that. (There's a handful that could)