Why is pawn forward not a good immediate defence against Bishop B5 / G4?

Why is A6 or H3 (pawn forward one) not a good immediate defence against Bishop B5 / G4?

In all strategy recommendations, it's always not the first response for pawn to put pressure on. Why is this? 

Sometimes it's a good move. Here are two common openings

In other cases it's usually not a terrible move, but it's usually not played because there are usually better moves.

In particular in the opening it's more important to bring your knights and bishops off the back rank and castle.

Also it's probably worth mentioning that if moves like h3 forced the bishop to capture your knight, then it would often be a good move... but often the bishop can retreat and keep pinning the knight. To break the pin with two pawn moves is doubled edged because it damages your pawn structure or just moves the pawns closer to the opponent... and pawns need pieces to support them, so again, in the opening it's usually better to first bring out your pieces, and then move the pawns later.

Ah thanks that does make some sense. I never understand why Bb5 is a good move though. Looking at your first example, pawns can just push the bishop up (e.g. to A4) and then again with the other pawn (B5) causing the bishop to retreat. Is there anything I'm missing here? Surely it's safe to keep bishops on the 4th row? 

Sure, Black can play b5 to push the Bishop to b3 and that very often happens in the course of the game. But the white Bishop is still fine, Black didn't win a tempo for development and the a6/b5 structure is double edged at best.

And yes, putting the white Bishop on c4 instead of b5 is fine.

A lot of people would pretend to know why each move is played.

They'd tell you it's a nice balance of developing and making threats. They'd give some reasoning for each move and ground it all in that context (attack and defense with development). For example they might point out that b5 is delayed until white has played Re1 because when the bishop is already on b3 white can play without Re1 so white would save a tempo on some lines and that's why black waits. He waits with Be7 because by putting a piece on the e file he shields his king and makes the threat on white's e pawn real.

But the truth is there is no a priori reasoning that will explain why it's good. Over hundreds of years people have played all sorts of openings, and the Ruy / Spanish happens to be one of the good ones

There is no fundamental logic that will make good openings intuitively good, and bad openings intuitively bad. The opening principals come close, but other than that you need to get a lot of experience by playing and looking at games.

That's the honest answer

And yes, the Italian opening (bishop on the 4th rank) is completely fine, and popular at every level (from beginners to GMs).

@llama44 I'd never pretend to have a deep understanding of the Ruy Lopez

I just wanted to point out that the possibility of ...a6/...b5 does not make Bb5 a bad move, because it's not clear at all that this structure is good for Black.