The 'Dark Arts' in both games - while ultimately stemming from a lack of sportsmanship - are aimed at obtaining an illicit advantage.
In Soccer players are not generally penalised for pleading or arguing with the Referee - so an appeal to authority is the name of the game. In Soccer it involves performative diving in or around the box to try and earn a penalty, or get the marking player carded, all as a show to the Referee.
'Selling' falling fouls are a particular result of the 'advantage' rule however. Advantage means that if you are making a drive towards the goal and are fouled, but you stay on your feet and are continuing towards the goal, then you are deemed to be playing on the 'advantage' and the rules require the ref to not call the foul.
Thus, in real cases of foul, if you fall and exaggerate, the odds of the ref considering what they saw to be a foul are higher. The more you ham it up, the more likely thereafter that the Referee considers it suitably egregious to award a yellow or red card against the instigator. Even the top technical players of all time like Messi and Neymar are notorious for this.
Given there's no negative consequence to trying it on in Soccer, people will milk 30 seconds rolling around the ground to run down the clock or allow physios onto the pitch or the team to otherwise regroup. Sports like hockey have a specific penalty for this performative falling called embellishment.
Other examples of the 'dark arts' in Soccer include pretending to get elbowed in Aerial play, pulling on uniform to imbalance or slow a player, or even using your hands to keep a ball in play without the officiants seeing it. Even the best players in the world like Thierry Henry are not immune from doing this, as evidenced when France knocked Ireland out of the World Cup 2010 Playoffs due to Thierry Henry's handball in the 13th minute of extra time.
https://en.wikipedia.org/wiki/2009_Republic_of_Ireland_v_Fra...
In Rugby only the Captains generally communicate with the Referee, and the onus is on polite and respectful dialogue and obeying the Referees word as final. Therefore in Rugby its less about gaining advantage with the Referee and more about disadvantaging the opponent. Thus they do things differently - things like raking players when on the ground - the deliberate scraping of an opponent's leg or arm with the studs of boots to cause injury - and biting, gouging, headbutting in the Ruck, pinching or punching when tackling, and thrusting when handing-off players so as to effectively punch them.
One may come off as more machismo or honorable than the other, but both are simply signifiers of poor sportsmanship and attempts to circumvent the mechanisms that attempt to make the games in question as equitable as possible. If anything, seeing the likes of Brian O'Driscoll get spear-tackled vs Arjen Robben chewing scenery all around him, the Rugby equivalent is exponentially more damaging to the players and the game.
https://en.wikipedia.org/wiki/Spear_tackle