In Bb5 systems against the Sicilian, why does White exchange their b5 bishop without playing a6? Isn't White giving up too much, i.e. one tempo in the opening and the bishop pair, for too little, i.e. doubled pawns?

For example, after

[FEN ""]

1. e4 c5 2. Nf3 Nc6 3. Bb5 g6



Bc6 is the second option in Lichess Masters database.
Some things that are probably part of the answer, but probably not complete and concrete enough:

1...a6 won't come. There is no threat to a pawn on e5, the knight isn't pinned, the only point of 3.Bb5 is to exchange it on c6. So black doesn't waste a tempo on forcing white to do what he was already going to do.

On the other hand, black has a choice to make, between ...bxc6 and ...dxc6. By taking earlier, white can see which way black goes and set up his pieces accordingly. If, in contrast, he takes later, black can first see where white puts his pieces and then choose which way to take. So since it's going to happen anyway, better do it immediately and not give out more information just yet.