What makes you think that all restaurants use the same exact mixture of beef and do not add any other ingredients?
The ground meat used in hamburgers varies from place to place. As an example, one restaurant might use just ground round and another might use chuck. Then there’s the difference in the amount of fat used in hamburgers. One place might insist that all it’s ground beef have 80 percent meat and 20 percent fat, while another might prefer an 85-15 ratio.
While a McDonald’s doesn’t season its meat, your local diner probably does.
And then there’s the method of cooking the burger. One of the easiest ways to ruin a burger is to press down on it during cooking. That gets rid of the fat, and the fat is where the taste resides.
One thing: I cannot imagine why you are unable to tell the difference between a McDonald’s hamburger and a hamburger from what you describe as an upscale restaurant.