Why isn't line to line voltage zero?
The difference between +1 and -1 is 2 yet both have the same magnitude of 1. If you connect two identical 9 volt batteries together with a single wire and measure across the unconnected terminals you might measure 0 volts or you might measure 18 volts depending on how you connected the single wire. Polarity matters and it matters in 3 phase systems just the same.



As you can see, although the 3 individual phase voltages are rising and falling identically, they are displaced in time and therefore there is a voltage between any two.

Picture stolen from here