Newbie question from checking out good players games

This is a puzzling difference I notice between the games I play and the games of good players.

In my games, pawns that are "in the firing line" - i.e. in a position in which another pawn can take them are almost always exchanged, whereas good players seem to often leave them there for ages without taking.

Why is this?

This is a complicated question which defies a simple answer. I might start by saying that multiple pawn exchanges tend to open up the game, and this can lead to lots of piece exchanges and a quiet, "drawish" position which is hard to get excited about.

 Consider the "Exchange French," for instance: 1 e4 e6 2 d4 d5 3 exd5 exd5. After this exchange of central pawns, the king file is open. The control of an open file is a strategic asset. Therefore, players will fight over the control of this territory by lining up their queens and rooks on the open file. This tends to lead to an exchange of these "heavy pieces." After a lot of pieces have left the board, it's hard to whip up an attack; instead, you're often left hoping your opponent makes a mistake.

Every exchange of pieces should have a point; every exchange of pawns should be considered just as carefully. Remember, as one grandmaster once put it, that chess is not checkers--captures are not mandatory!

Also sometimes a player waits for the opponent to commit to a move before doing a pawn exchange for example in some lines of the Queens Gambit and Slav when Black wait for White to play Bd3 and THEN plays dxc4 so that White cannot play Bxc4 in one move and must waste a tempo. Other than these subtle nuiances there also tactical subtleties which are invisible to the trained eye which could make it better to exchange at a certain moment.

Other times a player might be aiming for a certain pawn structure...for example a player might be aiming for an IQP position and thus chooses whether to exchange pawns based on this.

There are numerous reasons... 

Any specific examples?  They may be wanting to avoid doubled pawns, or isolated pawns.  There's generally some small advantage one way or the other from a pawn exchange, often going to the side that did not initiate it. 

You must have some awareness of that though, you have a higher rating than me.  And i'm generally regarded to be one of the world's finest players.

Here are a couple examples.

 There are a few other examples I could show but Im tired :) And have to get up early to coach tomorrow.

for an alternative to this: Read about Wilhelm Steinetz. the 1st world champion. He has these two famous sayings that are reflected in his chess play.

1. "When in doubt, take the pawn."

He was notorious for accepting all gambits offered.  Note that taking a poisoned pawn is not the same as accepting a gambit.

2. "The King is a strong piece, [that can take care of himself]." 

I regularly find the first part of the quote, but i've seen the second part somewhere.  In chess books that discuss heroic defence where the King becomes actively aggressive in middlegame or when the King goes on a walk through the Valley of Death from kingside to queenside or vice-versa... chances are ....it will be a Steinetz game

One of the jumps in ability I had was when I realized that pawn captures are not obligatory.  In fact, I'll now use it as a way to judge a player's relative playing strength.  In quiet positions, when there are multiple good moves depending on what your plan is, I will often perform a pawn break just to see if my opponent will capture or not.

Now, the main answer to your question, as many people have already pointed out, is that pawn captures open lines.  Not automatically capturing a pawn is known as "maintaining the tension" in the pawn structure.  Once that tension is broken, all sorts of tactics are bound to begin.  Before you make a pawn capture - or any pawn move, really - always ask yourself what lines are opened, whether or not your opponent will be able to make use of them, and whether or not you can contest them.  Often, the best move would be to bring more forces around to make use of the lines that are about to be open, rather than opening them for your opponent to immediately use against you.