Let's look at some of the motivations for cheating in its various forms.
One of them is one of the seven deadly sins: greed. Improving player stats is a great way to get a more lucrative contract, so a player may decide to cut corners. I will not deny that professional athletes work hard all their lives to enter the major leagues in whatever is their sport. However, the idea gets into some of them that by using artificial (and often illegal) methods to attain even higher levels of performance.
One of them is the system. Part of the earliest stages of the system is Little League Baseball. To begin achievement at that level takes two things: parents who will either participate in a player's training, or will invest in someone to teach them the fundamentals of the game. Here is where the parents also can do some of the most psychological damage. If you've been to a little leage game in recent years, or read the police reports (in newspapers that actually report them), you may notice on national scale an uneasy trend of parents physically abusing umpires and coaches for game events and play calls. For children of little league age, it is unconscienceable that they have to witness such behaviour from adults. But it also gives them a notion that if things go well, parents will be happy with them. So they go to great lengths to make sure that they do well, at any cost.
By the time they reach high school, or that age group in AAU organizations, the notion of win at any cost is so ingrained that they start using all kinds of extra substances (including steriods, presumably to increase muscle mass).
Here is where a glaring difference appears between professional baseball in the US, and professional football and basketball. Have you ever noticed how few baseball players come out of college teams? More now than in the previous years of baseball, but still a relative minority. It seems that this is because once a player goes into AAU or other high school level program, if there is even a rumour of them going to college for an education, they would be dropped from the program they were already in. And don't even mention the central and south american recruiting systems that have burgeoned in the past twenty years. Can you say, right now, without looking it up, how many players from college systems were in the 2007 All Star Game (not even mentioning their names)? I can't.
In many of these minds, cheating will get you there. Once you're there, cheating will get you more. And a side question related to that last segment: how many players named in the Mitchell Report came out of college systems? I will not say with certainty, but I will hazard a guess: none.