I'm running a business where I need to know who you are, because my platform can be used defraud other people. If you're trying to hide who you are from our very first interaction, that's a massive red flag.
If you can trivially create hundreds of these emails, and fill in the rest of the required info with bought/stolen/generated PII, now I have a vector for mass fraud. Requiring you to use a recognized non-anonymized provider doesn't stop you, but it sure does slow you down. (It's not this simple of course, but all security works in layers)
If these terms are not acceptable to you, then great! Don't use the website, there's no need to be salty because that's what you said you wanted. Isn't it?
I don't mind either, because the number of legitimate users who are bothered by this restriction is infinitesimal compared to the number of fraudsters who would take advantage if it wasn't in place. It can be difficult to comprehend the scale of platform fraud unless you've worked in this area, many days fraudulent signups outnumber legitimate ones.