What external identity information that is immutable would you like them to use? Your state or government issued ID? Because those change over time as well. Your physical address, that changes too. 2FA app on your phone sometimes those get lost or broken in an unrecoverable way as well. Perhaps submitting a DNA sample would be sufficient identity information that doesn't change over time.

As far as I can tell there is no perfect identity information that is immutable across time. For things like this you must keep the information up to date. It is unfortunate that there is not a centralized management or brokerage for this kind of thing but having a centralized identity management system comes with another set of problems whereas you have one system to breach instead of dozens of systems.

Steam does support email addresses as a second factor login and their app. It is always wise to periodically check that your identity information on every platform is up to date especially if you've moved or changed phones or things of that nature.