They would need to make some tie between sasl authentication and what FROM: headers you're allowed to use. I don't know what MTA they use, but there are MTAs that have that feature. It's controlled_envelope_senders in postfix. I assume other MTAs have similar features.

It sounds kind of lazy to me. Though I'm sure they would get lots of complaints if they turned it on...some mailing list software depends on spoofing, for example. Or web based "contact us" forms. So perhaps it's just to avoid lots of support tickets.

The reason is probably that nothing can stop the successful spoofing of the From header. DKIM is a signature for authenticating a domain, however that domain does not have to match the domain in the From header.

Take a look in Gmail at a signed email and you’ll see a “Signed by” field in its header info, with a domain name as a value.

Also the SPF setting has nothing to do with the From header either.

In other words the “From” value cannot be protected, unless you sign your email with PGP or S/MIME.

I'm not certain, maybe there's a technical reason they can validate account credentials but not map credentials to addresses/aliases. Doesn't instill confidence either way.