Bullets travel in a straight line too but that doesn't make it as fast as light. They don't go out in a continuous beam and they literally look like
this. They may be made of light magic and-or emit light, but they are not a
beam of light.
The idea that the Light Arrows aren't real light, just arrows imbued with light is even confirmed in-game several times:
"You got the Light Arrow! Set it to (C) to
power up your arrows. The light of justice shall target evil!""
"You got the Light Arrow! Set it to (C) and
your arrows will be
powered up! The light of justice will smite evil!"
"You got the Light Arrow! When you take out your bow, use [R] to
change arrowheads, then fire away. The sacred light of these arrows can pierce pure evil itself!"
"You got the Light Arrow! Now, you can fire
arrows filled with sacred light!"