Better question is why didn't the Olympic singles gold medallist show up to defend his medal in 2012 ? Please don't give me the 'he was injured' argument. Just until one month before he was beasting and having one of his best clay season runs. A loss to Rosol couldn't be so bad to digest that he had to bail out of Olympics.
Nadal did give a

explanation for missing the 2012 Olympics. But with Nadal, it's always wise to check his words against his actions.
Nadal claimed that his left knee had been injured since February 2012 (hoffa syndrome, partial tear, tendinitis). These are overuse injuries, i.e., they happen because the player played too much and over-strained his body. Yet Nadal continued to play a packed schedule of 9 tournaments from Indian Wells to Wimbledon (he even added extra matches by playing unnecessary doubles at Halle, Indian Wells, Miami).
Nadal wasn't taken out of the Rosol match on a wheelchair.
He was walking and running normally to the end, except for moments when he was outplayed and began looking anxious and resigned. If Nadal looked fit enough at the end of the Rosol match, why would he not have become fitter after four weeks of rest before the Olympics?
Why couldn't Nadal have made the effort to play the Olympics at Wimbledon, especially if the gold medal really was "that" important to him. After all, in 2005 Fall, Federer had suffered ligament tears in his ankle, was in a cast and on crutches for two weeks, yet one month after getting injured he showed up at the year-end championships in Shanghai where he lost a five-hour, marathon five setter against Nalbandian even though
his ankle handn't fully healed.
Also, a person with knee injuries should not be playing golf. However according to *******, himself, he
played golf almost every day he was off - from at least
early September 2012 to late January 2013. Now, remember that Nadal plays golf right-handed, which means his front knee is his "ailing" left knee, so the front knee in golf is the one that receives all the
torque from the downswing. It OBVIOUSLY makes perfect sense that an athlete in fear of his career due to a left knee injury would play golf every day while "recuperating" from his "injury."
It's rational and reasonable to conclude that Team Nadal did not want to risk Nadal losing to Federer, Djokovic or Murray at the Wimbledon Olympics after his shaky form on grass (given Nadal's first round loss in 2013 Wimbledon, this is plausible). It's also reasonable to consider that losing so early at 2012 Wimbledon, given the points situation then, meant that Nadal was unlikely to finished the year No.1 - thus #TeamEgo shut down for the season.