We hear all the time the question, "Why would they dope if they know they will be tested?" Let's look at the record.
  • Tyler Hamilton doped for a full season or more but only failed one (well, one and a half) tests. Same with Perez.
  • Heras appears to have doped for a long time before he was caught (see Operacion Puerto).
  • Millar doped, but always tested clean. He was only caught because empty vials of EPO were found in his house.
  • The Festina boys all tested negative.
  • Basso, Ullrich, Gutierrez, Mancebo, Sevilla, and many more were tested over and over, but were never positive. They were only caught because of Operacion Puerto.
Basically, in order to avoid false positives the anti-doping tests allow a very large gray area where it's likely that the cyclist doped, but not certain enough to call foul. Many cyclists appear to take advantage of this gray area to dope at low dosages without being caught.

The people who do get caught are those who are careless or unlucky and stray from the gray area to where the test is definitive. The fact that most dopers manage to pass dozens of tests with flying colors probably gives them confidence that it's safe to dope as long as you follow the right protocols.

There is ample evidence that many cyclists can dope regularly without being caught so, apart from ethics and sportsmanship, why wouldn't a pro dope just to remain competitive?

So someone might be confident that if he took only enough testosterone to get T/E ratios of 3:1 he would be safe from anti-doping tests, so he passes test after test throughout the season until one day things go wrong and he goes way over the 4:1 threshold and is caught.

And when he was caught, knowing that most of the peloton was doping and he was one of the few unlucky enough to be caught, wouldn't he feel outraged and unfairly treated?