Unfortunately, the people abusing these are very persistent and adding more checks will just increase the sophistication of such attacks which works against the interests of the target.

This stops disguising yourself as a legitimate service.

If they did that you’d decry censorship.

Allowing basically ~whatever, but proving the sender address is an obviously good approach to me.

email is weird, without this it doesn't perform the checks at all

Agreed. There's no additional verity provided by adding a DNS record with a static line of text. Reminds me of airport security theater: meaningless hoops for citizens to jump thru under the guise of stopping bad actors.