In the beginning of The Phantom Menace, Obi-Wan Kenobi and Qui-Gon Jinn were sent by the Supreme Chancellor of the Republic to discuss a settlement to stop the Trade Federation from blockading the planet Naboo, but were attacked by the Trade Federation on the orders of Darth Sidious. After surviving the attack, they managed to escaped to Naboo by stowing away on a Trade Federation invasion ship heading to Naboo. The invasion army was made up of battle droids, so there shouldn't have been a need to pressurize the cabins, or pipe in oxygen. How, then, did they survive the trip to Naboo?
While a robot may not require oxygen to function, that does not mean it is vacuum-compatible. Many materials are perfectly ok at normal to low air pressures but will outgas at near-vacuum pressures. The consequences range from nothing to popping open like a bad tire. Lubricating products are particularly troublesome, and we don't want the grease boiling away and rupturing the joints. The item also has to be free of sealed internal voids or the shell of those voids needs to be able to contain the pressure difference. This is obviously much more difficult and expensive to design.
The same would apply to the cargo area of any transport vessel. Parts of the ship that must be exposed to vacuum are designed as such, other parts get the cheaper treatment.
So, if you are designing a machine for use exclusively within an atmosphere, it is simply easier to transport it within a compatible atmosphere.
The film's tie-in "Incredible Cross-Sections" book states that the design of the landing craft wasn't bespoke, it was based on an earlier design for a cargo ship, one intended to be staffed by a living, breathing crew.
That being the case, trying to vacuum-proof a ship that was originally intended to contain an atmosphere would present a number of fairly obvious design difficulties
It took about an hour to land the Space Shuttle; given Star Wars technology I have to assume the invasion ships could do so far more quickly. According to these calculations a single cubic meter of air will last one person for two hours, so assuming the ships were pressurized there would be more than enough air.
Why would they be pressurized? Well, for one thing, the mother ship was pressurized (presumably due to the presence of all of those pesky humanoids) so if you wanted to load the landing craft from the mother ship it would be much more convenient for them to be pressurized too. Also, the landing craft might sometimes need to be piloted by or to carry humanoids, so they need to be designed to be pressurized; having them pressurized only some of the time would probably complicate the design.
Even more importantly, as Royal Canadian Bandit points out in the comments, if the ship held vacuum you'd have to repressurize after landing before the droids could exit. In the meantime, they'd be sitting ducks.
I think temperature would be a larger concern. There's no reason to heat a ship with droids (presumably you would dehumidify the ship to prevent condensation). Granted this is Legends, but Kyp Durron ejected in a message pod and was subjected to the extreme cold of space (short term), but survived
Durron realized that he would be unable to escape the black hole and rushed to one of the craft's message pods. Utilizing the Force to deaden the vast pain, Durron crammed himself into the message pod, the inside of which could barely hold a child. Despite the breakage of many of his bones, including those in his limbs and ribcage, Durron managed to squeeze himself in to the message pod and placed himself in a hibernation trance.. The pod fired away from the doomed Sun Crusher as the superweapon spiraled into the black hole.
When they discovered Durron's broken body in the message pod, Solo and Calrissian were shocked at the extent of Durron's injuries and pried him out of the small container. The young Jedi opened his eyes long enough to check that the two superweapons had been destroyed before sinking back into his hibernation trance to heal.
We already know canon Jedi can force themselves into a state where they breathe less. I don't think it's unreasonable to assume a canon Jedi could survive a short term temperature drop.