Doesn't it? It gives me two boxes, "current location" and "destination" and I can change either.

I can't reply to eitland for some reason, but yes, I get both those boxes.

I open the app, click my destination, and then click "Directions". The very next thing is both of those boxes, with "current location" defaulting to the start location. I can then change that if I want.

It optimizes for my most common use case, but allows me to do it otherwise, too. I don't think I could design this better.

Straight away after you open Google Maps on a mobile?

No, I open it, look for the destination, then press directions and can edit the starting point.

Then we agree. I find that utterly annoying since I've seen how simple it could be but it seems many people disagree with me :-)