What is it about sci-fi that if you don't see a body their probably still alive?

I'm pretty sure Jien Garson's body was "shown" in Nexus Uprising. As for the Jardaan, we have no information on them as to whether they are dead or alive. All we know is that they aren't around. Their fate is unknown. Alec is the only one said to be dead with no body shown. I think it's odd that there's no memorial or anything at all to mark his passing. To me, that's odd. Sure, it may just be a flaw in the writing, or something they opted to cut out for time reasons, but it's strange.

It's a general truism for a lot of genre fiction, of which sci-fi, fantasy and crime stories/thrillers are the most popular these days. Because they have a tendency to rely on twists and turns to keep the audience's attention, characters improbably returning from the dead is a very popular trick. However, it has been used so many times now that many fans have caught on to it, so if there's not at least a bloody corpse, people often spin theories. Also, authors may just not want to close certain doors for themselves. (This is not to say more highly regarded literature and art movies or artsy indie games never pull this stunt, but generally they tend to rely less on the 'aha, but I was alive all along hiding in the basement!' kind of plot twists, since they can sometimes be a bit hoakey.)

The Jardaan are definitely coming back, I think, at least in some form, if/when the game gets a sequel. We've only just scratched the surface of their story. Of the seven in the Ai leadership that were before "Number Eight", Tann, we might also not truly have lost everyone yet. Alec is more difficult, but as mentioned above by dmc1001 , not impossible. However, I don't really see that he's necessary to the plot anymore, which is really my main reason I think we won't get him back, unless they involve him deeper in that Benefactor business or set up something else for him connected to the reason he faked his death.

Meh, that's not a sci-fi thing, that's a fandom thing. People do it in Dragon Age as well with characters like Duncan, enough that David Gaider had to confirm that, yes, he was dead, and no, he's never coming back.

As for its use in storytelling, especially in serialized format, it is a useful tool for creators to use in case they want to bring the person back for whatever reason. They're leaving their options open, as it were. But sometimes fans get carried away with it, defying logic just for their own wish fulfillment.