I figure they never needed a complex motivation, and what we saw in ME2 would have sufficed, much like how you don't exactly need to justify the actions of the Replicators in Stargate SG-1. Their replication process provides a justification of their actions, abeit a simple one. On the flip side, sometimes a simple justification can be incredibly profound.
Why climb Mt. Everest? Because it's there. Why do the Wraith hunt humans? They eat them. Why do the Dukes in Trading Places set the plot in motion? They made a gentlemen's wager. And then we have the more mysterious motivations...Why does the villain of Majora's Mask try to destroy Termina? Who knows? The characterization we're shown though does seem to indicate it was just for the heck of it though. The villain of Persona 4 (not the secret one)? Same reason, or at least that's what he claims. Why is the villain of Halloween doing what he does? Well, originally, he wasn't given a motivation at all[footnote]To the point that when the actor himself asked about it for a scene, the director responded that his motivation was "to walk from one set marker to another", and he's even simply referred to as "the Shape" in the credits. Carpenter's own descriptions of Myers simply label him as a 'force of nature, an evil force that's loose'[/footnote], which made him all the more terrifying. Why does Ledger's Joker do what he does? ...Well, Alfred suggests that he's just the kind of guy who likes to watch the world burn, Batman posits that Joker wants to prove everyone's as warped as he is, Joker himself seems to suggest that he just likes the act of doing it ('a dog chasing cars', as he put it), but ultimately, we don't know, and all the same it is regarded as one of the best interpretations of the character to date.
The villain's motivations don't need to relate to the concerns of the protagonists, especially if they exist on the scale or alienness that the Reapers embodied, to say nothing of their own perception of their superiority. Given those circumstances, I'd actually say explaining things about them cheapens their nature, and in this case the problem is exacerbated by the attempt to portray the Reapers - the villains of the series - as sympathetic and (by virtue of Shepherd's apparent acceptance of their logic) the new effective protagonists of the series during the series' climax.
Why climb Mt. Everest? Because it's there. Why do the Wraith hunt humans? They eat them. Why do the Dukes in Trading Places set the plot in motion? They made a gentlemen's wager. And then we have the more mysterious motivations...Why does the villain of Majora's Mask try to destroy Termina? Who knows? The characterization we're shown though does seem to indicate it was just for the heck of it though. The villain of Persona 4 (not the secret one)? Same reason, or at least that's what he claims. Why is the villain of Halloween doing what he does? Well, originally, he wasn't given a motivation at all[footnote]To the point that when the actor himself asked about it for a scene, the director responded that his motivation was "to walk from one set marker to another", and he's even simply referred to as "the Shape" in the credits. Carpenter's own descriptions of Myers simply label him as a 'force of nature, an evil force that's loose'[/footnote], which made him all the more terrifying. Why does Ledger's Joker do what he does? ...Well, Alfred suggests that he's just the kind of guy who likes to watch the world burn, Batman posits that Joker wants to prove everyone's as warped as he is, Joker himself seems to suggest that he just likes the act of doing it ('a dog chasing cars', as he put it), but ultimately, we don't know, and all the same it is regarded as one of the best interpretations of the character to date.
The villain's motivations don't need to relate to the concerns of the protagonists, especially if they exist on the scale or alienness that the Reapers embodied, to say nothing of their own perception of their superiority. Given those circumstances, I'd actually say explaining things about them cheapens their nature, and in this case the problem is exacerbated by the attempt to portray the Reapers - the villains of the series - as sympathetic and (by virtue of Shepherd's apparent acceptance of their logic) the new effective protagonists of the series during the series' climax.