Perhaps it's harder to justify internally if the only people watching know exactly what you just did? I've always been surprised by the shamelessness of soccer dives, I definitely don't understand it.
A totally subjective answer here, but I think it's because of the speed the game is played at.
It's very easy to trip when sprinting at full speed while concentrating on a single object. The slightest external force will put you on the ground very quickly.
Rugby, Gridiron and Ice Hockey players will also know that, but frequent falls under pressure are expected by the rules. In soccer, it's a lot more subtle (shirt pulling, various rules around obstruction, the height at which you keep your hands... all sorts). There's plenty of opportunity fool a referee, so you will instinctively try to do so. Especially if your style of play generally draws opposing players into fouling you regularly already.
I'd say that the introduction of video referees (VAR) are probably more effective in stopping that kind of behaviour, than empty stadiums. But you'd never know.
