It sounds like you are trying to shoehorn email into some kind of “real person verification” role, when you ought to be doing actual KYC through some provider like ID.me. (If honest to god no-shit fraud is on the table.)
If I can filter/throttle fraudsters at the create account step for free, I save on the fees my KYC/IDV providers charge each time they attempt to defeat it.
At the cost of blocking legitimate users who don’t want to be spammed, don’t want to be correlated after a data breach, etc.
I have been willing to do KYC for services (usually financial) without giving out my main email. Services that put up too many barriers to this don’t get my business. I concede that there aren’t that many users like me, compared to the general public, but I’m a legitimate user.