Ummm, to keep them from dying. Don't be a jerk. Like any sports or entertainment business, there is pressure to get to the top and part of the pressure concerns either looks or athletic performance, thus some of the wrestlers choose to be roided. Let's also remember that given the hellish travel and performance schedule of a high-level professional wrestler, steroid use to aid recovery is not a bad idea.
I watch the hell out of WWE, so I have a few thoughts.
The death of Eddie Guerrero is what brought on the supposed drug testing policy. Eddie died of heart failure. While he had previously had problems with painkiller and alcohol abuse, he had been clean for four years. However, it is entirely possible that he was using or had used steroids. At the time of his death Eddie, at 5 feet 8, was billed as 228 pounds. Prior to his rise to the main event in WWE, he was much smaller.
I haven't noticed much generally in physique decline for many of the wrestlers. The only wrestlers noticeably flatter or smaller are those who were close to Eddie personally: Chris Benoit, Batista, Rey Mysterio. Rey Mysterio in particular seems to have a rapid decline in body composition. Batista's loss of size may be attributable to lack of lifting stemming from his recent torn lat. Interesting to note, however, that the physiques of both Kurt Angle and Chavo Guerrero are much the same since Eddie's untimely passing. Chavo may even be gaining size, which would be understandable if he were looking at better future prospects.
Other wrestlers seem not to have changed at all, especially those whose characters depend on their physiques, such as Chris Masters, or those whose spot on the card is not so secure. In fact, Masters does not have my favorite physique in the WWE. Bobby Lashley does. Lashley won the NAIA wrestling championship at 6 feet 3, 177 pounds. He is now billed as 273. He is swoll as hell, and he has not lost any size in the last 2 months. Other wrestlers of note that look no different include Rob Conway, John Cena, Kane, Booker T, Randy Orton, Val Venis, Undertaker, and Shelton Benjamin. So then, these wrestlers were either never roided, or remain roided in spite of the testing policy. I have no idea which case applies to which wrestler, although Cena and Orton are on record as claiming to have never used.
At the very top of the card, note that Shawn Michaels and Triple H are for sure not on anything...because we can remember fondly how they looked when they were. Triple H in particular, now that he is a main event fixture at any time, has gone from a sick physique 3-4 years ago to kinda fat-swoll nowadays.
My thought is that the policy applies mostly to prescription and recreational drugs rather than roids, because the roids can only be eliminated at a cost to the product. Honestly, it's pretty obvious to WWE that we like watching "larger-than-life" wrestlers. So what does it mean if Vince McMahon has been in matches before?