It's mostly that ground meat (of any kind) is all surface area and surface area is where the bacteria is. With a steak you can eat it rare/raw in the middle because the surface area is just the outside and it gets the heat which kills bacteria. With ground meat there is a fuck ton more surface area, all the way through the patty.

You're right if you grind your own stuff or eat at a place which does it right before cooking, you're much safer. It's not a guarantee though.