Just because the alert signal is an email at one point in its journey from sensor to human doesn't mean you're relying on someone using a computer to see it. Automated paging systems can read email too.
You’ve got a chain of multiple single points of failure. That’s unacceptable.
Yes, but we have to remember email is not guaranteed to be delivered, it's not reliable. You cannot in general tell if someone even looked at it (people try to build systems on top to provide these things, but it's not universal).
Also, I have a personal domain, some sites decided its not real enough and I can't get email from them.
Email is just not reliable. It could be part of a combined system of other things that are reliable.