Why lateral pawns rarely advance to defend from bishops?

Hello everybody! 

Usually strong players do not use the lateral files pawns to defend from a bishop attack, but I don't understand why. For example yesterday I saw the beginners video about the openings and it used the Ruy Lopez opening to present some concepts (I put it at the end of my post). However I can't understand the move Bb5 of white. I see that usually pros do not use moves like a6 to get rid of the bishop, but at my level this kind of moves are very common. I used the Ruy Lopez opening as an example and I know I can't understand it fully at my level, but I could have used a lot of my games, because it always happens that when I advance with my bishop the opponent uses his pawns to get rid of them. Submitting games to Fritz for analysis was useless, because he suggested those advancements with bishops and never considered the fact that my opponent could advance with his lateral pawns, so I suppose there's a reason why strong player don't do this. But why?

I believe a6 is good if you're going kingside castle for sure. if something occurs, and you need to transpose into queenside castle, a6 will further weaken your pawn structure. Using the example you showed, the missing b7 pawn can easily be recovered with Bb7 (if bxc6 did occur, which will weaken your queenside pawn).

a6 is the most common response. The bishop just moves backwards, and if b5 the bishop moves to a good square.

Play can also continue 5...Be7 after Ba4, white castles, black plays Nf6 and after Re1 the threat of Bxc6, dxc6, Nxe5 is renewed as black is supporting his own pawn so black will now play b5.

The move 3...a6 is attributed to Paul Morphy. Sometimes it is used in conjunction with 4...d6 which is called a Modern Steinitz Defense.

You are right to wonder Why place a Bishop where it can be chased with pawns. There is a general rule that Bishop moves to pin Knights are stronger when played after the opponent has castled on that wing, so that pawn kicks at the Bishop are weakening to the king position, rather than being welcome advancements.

However, the Ruy is just one of those exceptions where WT allows his Bishop to get kicked, knowing that even tho that helps BL get a Q-side attack going ... white will still get to attack on the King side, and/or Center. And believes his will still be the stronger attack. That belief is not unfounded either, according to a survey I once did of decisive Master thru Grandmaster games during the 70's, for instance, the Ruy had a 70% win score for White. Easy to see why it was so frequently played by Fischer and other GM's

thanks a lot to everybody, your answers have been really useful!!!