That, and although water and earth are conductors, they're relatively poor conductors and can still set up a situation where a swimmer becomes a current carrier, which is far more dangerous in a pool than anywhere else due to the reduced skin resistance, nature of the circuit (current is very likely to pass through the heart), difficulty of breaking contact, high potential to drown, and difficulty of rescue as anyone trying to conduct a rescue would be at the same risk.
The equipotential bond ensures there will be no equipotential difference within the pool and pool area, so that even if a short were to occur, the entire pool will be at the same potential and no dangerous amounts of current can flow through the water.
