F9 boosters don't return from orbit or more importantly orbital speed. Re-entry heating goes up with the cube of speed so twice as fast, 8 times as hot. So the F9 boosters can return engines first. Starship doesn't have that luxury as it's coming in much faster. It needs a heat shield and the larger area the better so they came up with the unique idea of using half the vehicle and covering it with tiles.
For these initial tests they don't need the heat shield but we will prolly see after the next vehicle.