It's difficult to say why Face ID doesn't open a phone without requiring a further swipe but I'd speculate at two possible reasons:
1) "Unlock phone using face recognition only" has been patented and Apple don't have a licence for that patent. It sounds silly but Apple patented unlocking a phone by typing in a number only on a numeric keypad. As a result, non-Apple phones also have to hit the enter key after entering the number.
2) Accidental unlocks. If you pick up the phone and press a button or touch the screen it will trigger the Face ID process. The swipe is a positive confirmation your intention, ensuring that your phone doesn't stay turned on when all you (thought you) did was pick it up and put it in your pocket.
Having the option to turn off that step would be nice.
V.