Dear every airline, explain to me why I have to manually copy the booking reference and my last name from the email you sent me, the one that includes the link to the online check-in web form

I can imagine there is some law that requires „manual“ identification. I.e. something explicitly forbids to pre-fill the form.

Some e-mail spam protection gateways "click" links in e-mails, so perhaps that's part of the reason too?

Why would it? I’m not suggesting anything is done on the server yet on the GET, just that it pre-fills the form with the info already in that email

Are the number and your name already elsewhere in the email? If yes, then of course the link should include them.