Why do a lot of Americans have surnames as their first name?

McKenzie, Cooper, Dempsey, Miller, Mitchell, Paisley, Reid, Reagan, Rockwell, Quinn, Brook, Flynn, Murphy, McCann, Riley, Brooke, Austin, Russell, Taylor, Morgan, McCauley, Mackenna, Mackay. There are so many and can be confusing as a non American when you ask them what their Christian name is.

Why do a lot of Americans have surnames as their first name?
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