Well, yeah. But in Dune there weren’t entire ships dedicated to transporting the stuff across interstellar distances*.
*Your ship might be completely crammed with Spice, true, but it gets into a much vaster Guild ship which does the actual interstellar travel with your ship in it.
Remember in the first movie, when Padme is disguised as one of her handmaids, while the handmaid pretends to be her? They should have kept the maid as a minor character in the next two movies, and had her be the one who takes Leia to Alderaan to raise as her own.
Alternatively, they could have named her Beru, and had a subplot about how she’s always bickering in the background with Padme’s chief of security, Captain Lars…
When ANH came out, Lucas didn’t know 1) there’d be a sequel, B) Vader would become a major character in sequels, III) Luke and Leia were siblings, and 4th) Vader was the bio dad for Luke and Leia. So, Lucas didn’t have Obiwan lie to Luke in ANH.
Leia: Luke, now I feel very uncomfortable about that kiss we shared.
Luke: (waves hand) We never kissed. You don’t remember a thing.
Leia: What was I talking about?
The meta reason is that it was at the end of the movie. A surpring revelation at the beginning of a movie or in the middle might turn out to be false (and thus drive the story in interesting ways), but a revelation at the end of a movie will be expected to be true by the audience.
Also, Luke’s mom doesn’t really exist in the narrative of the first three movies. The only time she’s mentioned is when Luke as Leia about her mom in Jedi - before that, there’s not a single reference to Luke’s other parent, not even obliquely, in the entire trilogy. In most stories, if there’s a question about someone lying about a major plot point, the audience is going to assume that it’s one of the characters already on stage, and not some third party they’ve never heard of.
Sorry about that. I misunderstood the question. Miller has a good point though - if Luke’s mother was a character in the movie (or even a character that Luke mentioned from time to time) then the idea that she had cheated on Anakin with Darth would be a dramatic surprise, but without that, there’s not much point to it (IMHO).