In reference to this post at Daring Fireball - I have always reasoned that the cursor is placed at the top of the new message when you hit reply precisely because you will need to start at the top with a salutation, then work your way down through the message adding your interjections after relevant paragraphs and deleting anything which you don’t need to quote.
The problem is that these days nobody bothers with that, they assume the cursor is at the top because that’s where their reply is supposed to go.
The worst example is when you get sent an email with something like “What do you think we should do?” at the top, and you scroll down to realise that you’ve been sent a massive dollop of email correspondence going back three weeks, which you’re going to have to read from bottom to top before you can reply.