Why not fight off a bishop on row 5 with pawns? Oftentimes a bishop will move to b5 or g5 early in the game.



I personally never do this because I see a6 coming, and if I move the bishop back to a4, then b5, nearly trapping the bishop, and certainly having made it a waste of time to move it to b5 in the first place.

However, when I watch games by top-level players, bishops get moved to the 5 row all the time, and nobody ever fights them away with pawns like I think they will. Why not?
Here are a couple of relevant questions and answers that might help to explain why the bishop move happens:


Morphy Defence in Ruy Lopez
Why is the Ruy Lopez such a common opening?


In summary the main points seem to be that:


Black loses as much time chasing the bishop as White does retreating it
White can gain a long-term advantage from Black's weaker Queen side pawn structure


As you might expect in such a popular opening though it's very finely balanced for both sides.

If as you say you are not seeing a6 and b5 in the games you're looking at, you might be looking at games where Black plays the Berlin Defence: 3... Nf6. This is quite popular at the moment in very high level games. 

If you want to see some games where the pawn move a6 is played, "putting the question to the bishop", here's a link: http://www.chessgames.com/perl/chessopening?eco=C70