The door handles are probably a deliberate choice to get the drag down as low as possible, without the expense of motorized handles like the S or motorized doors like the X.

No other cars on the road have drag coefficients as low as the Model 3 (or the Model S, for that matter). From what I've seen, Teslas are the only cars that have the B-pillar flush to the glass, and flush, cavity-less door handles. That plus the perfectly smooth underbody nose to tail is what gives the Teslas better range per battery capacity than other electrics.