In both cases, the universes both novels create is too elaborate for a conventional 2-3 hour movie, so you need lots of exposition.
You can almost work out mathematically what the maximum number of protagonists for a normal 2-hour movie should be. For movies which are NOT adaptations from novels, you don't want to go over about half a dozen. Too many characters and you need to spend too much screen time fleshing out each one with backstory and motivation and personalities (that pass the Plinkett test). If 5 minutes of screen time is enough to convey backstory, motivation, and personality, then six characters gives you 30 minutes of character development, which is 25% of your 2 hour movie, before you get to action, comedy, love, set-back, and finale.
Now for Dune or Lord of the Rings, the novels contain dozens of characters and complicated elaborate backstories, and it's just too much for a normal movie. Hence the prologue. Luckily Lord of the Rings is an 11 hour movie (I don't think of it as three movies). And Dune is every bit as rich and needed the 9-11 hour treatment as well, or a TV series.