This is because Spanish b and v are both pronounced with a bilabial sound, which means the sound is made by bringing both lips together.
Examples:
* Baca (roof) and vaca (cow) are pronounced the same.
* Bebida (drink) and verdad (truth) are pronounced the same.
However, there are some instances where the sound of b might change depending on the context. For example, in the beginning of a word followed by an "m", b can sound like m. For example, "bomba" (bomb) sounds like "bomba".