> [CAPTCHAs] are a lot more reliable than an email which is basically just a domain that anyone can buy and setup within mere minutes giving them access to endless email addresses.

CAPTCHAs also can be unreliable with false positives and false negatives and other problems (although some CAPTCHAs are worse than others). "CAPTCHAs are trivially defeated today, regardless of the provider" is just one more thing, that makes even more worthless.

> Why not use phone numbers instead if the issue is truly important to them?

Even telephone numbers you might not have, or might be shared with someone else (more likely to have shared telephone numbers than with email).