Analagous to the decision for the Saturn-V to do "all up testing", with all three stages.
As opposed to the early Saturn-I's that only flew a booster and dummy stages.
That was a key reason Apollo landed on the moon in 1969.
THere's plenty of "discarded" Starship prototypes that could have been flown on top as a dummy, but they need to test as much as they can get done.
I'm disappointed they won't try to go from belly-flop and do a "flip" landing to get more testing out of it (only worked once). I read someone claim it was to assure SS would get crushed and sink, but if that was the reason they could just program it to flip and "land" 1 km above the ocean, reach zero velocity, then shut down, the velocity of falling free for 1 km woukd be enough to crush it and sink. Still, I found that "belly splash" reason to sound too much like a fanboi speculation presented as "fact", and not anything hinted at by SpaceX AFAIK (maybe legit, but.....).
What I had really really hoped, was they would set out a dinghy with a drone, near the landing spot, then have the drone take off 10-15 minutes before landing, and get to a spot to get a good view of landing. Mainly to check for damage like missing tiles and possibly melted/damaged steel sections of StarShip. SpaceX has used drones for other reasons, including RTLS landings at the Cape, but never at sea