They just did.

(Hint: potential users count.)

Synchronizing all the iphones in a room wouldn't be technically challenging assuming their clocks are all set at least approximately correctly, and you just make them flash colors/patterns that are generated deterministically using the system clock.

Actually this could look really cool!

There is no reason all phones would have the same time. I know some US phones sync the time with the carrier, which is a great function but not seen in the rest of the world.

You could use bluetooth, but then you have to switch it on.

You could use the camera to pick up the phones behind you somehow, but that would not always work.