Rebels apparently has no problem showing star destroyers in atmosphere. Disclaimer: I haven't seen Rebels, just came across this in an answer to this post:
https://scifi.stackexchange.com/questions/147527/how-could-a-star-destroyer-hover-over-a-city-in-rogue-oneAlso several points:
- Vader is shown as having to go meet the Death Star, and must have traveled there in his star destroyer, thus it wasn't at Tatooine anymore. He gives his men orders to find the plans, then rushes off for his important meeting.
- It's been a while but I don't recall Vader having an
additional star destroyer at Tatooine in the first movie, unless Lucas decided to retcon one in for the Special Edition movies, and in that case it would have been to explain how the remaining storm troopers planned to get home after Vader's star destroyer already left, so they're still left with
one ship, not two.
- Tatooine is a planet with apparent Earth gravity thus it's likely to be Earth-sized. Originally, the ground troops only suspect the plans
may be in the escape pod, until one of the stormtroopers reveals droid parts. They then track the movement of the droids on the planet, and
eventually this takes them to Mos Eisley. But they can't know ahead of time whether the droids are together or separate, which droid has the plans or that the droids met someone who even wanted to take them off the planet. There's too much
viewer knowledge in all that.
- also, what if the droids met someone with an X-Wing fighter? Small, hyperspace capable ships are shown to exist and be capable of taking off from just about anywhere, not just spaceports. The imperials can't have known ahead of time that the droids
must be headed to the spaceport, and we're not told that the planet has only one spaceport.
- Even if the star destroyer can enter atmosphere and knows for sure that it's going to be from Mos Eisley, they can't hyperspace from inside atmosphere, and small ships have been shown to be capable of out-maneuvering things as large as a Star Destroyer. Being in atmosphere might not be the cleverest tactic in a chase situation.