See the point of going Bf1 is not just to attack the a6-pawn.
White also wants to IMPROVE THE BISHOP's postion.
So after 1.Bf1! a5 2.Bc4, White managed to get their Bishop to an active square c4 -square.
On the other hand, after 1.Bb7 a5, White's bishop looks a little silly on b7. Sure it attacked a6, but what else is it doing on b7?
In this case, White will again have to reroute the bishop with 2.Ba6 and 3.Bc4 wasting another useful tempo.
That's why 1.Bf1 is better than 1.Bb7.
The goal is to give the bishop a better job while using the fact that a6 is undefended. :)