For soccer - There is evidence of Italian and British clubs using drug transfusions. There are videos of Cannavaro on Youtube while at Juve taking blood. Chelsea went public with reinfusing blood in the early 00's. There are testimonies of German internationals from the 70's being injected with everything and anything. If you can take EPO and be fresher in the 89th minute than the other team with no chance of being caught (remember, the blood testing is rare) then why wouldn't you?
For tennis if you can take steroids or EPO and know you have little chance of getting caught you can train 2-3 times a day instead of the once - bringing massive strength and phenomenal endurance to the court for the sake of a good show. If you are glowing and a tester arrives you can hide in your panic room, even though it is the time and day you told the tester you would be at your house and then tell the world you thought you were being robbed (Williams) and people believe it because they don't want to believe the alternative.
Rugby has the highest number of athletes serving drug suspensions in the UK. Players are having to take mid career breaks to recover from spending 80 minutes a week getting beaten up. At local amateur level banned substances are normal here, it's a level playing field almost.
Cycling isn't clean, it's cleaner but there are still suspicious performances. It is a handy whipping boy though. During the whole Olympic scandal last year the first question of the first interview I heard on the BBC spoon-fed the party line to the interviewee from British Athletics along the lines of "do you think you will become as bad as cycling?". How can you answer yes to that? Why would you spit in the soup and admit you have a problem when you can just bring cycling into it?
I feel very strongly about this as a sports fan in general who prefers Cycling. The hypocrisy is infuriating.