There isn’t any one reason, but rather a combination of factors.
It’s safe to assume that any spacecraft on it’s way towards imminent deconstructive lithobraking would do everything possible to counter gravitational acceleration. If the repulsor-lifts were at least partially operational and just not strong enough to completely counter gravity’s pull, you would hit the ground slower and with more of a crunch than a bang the same way that slamming on the brakes before you hit a brick wall will crumple the front end instead of making the bumpers meet. That lower speed alone will also reduce the amount of heat generated by plowing through the atmosphere.
However, all else being equal, mass increases by the cube of the volume. Assuming equal lifting power for a given volume of lifter, a craft the size of the Millennium Falcon will need to dedicate a larger portion of it’s internal volume for lifters strong enough to leave a planet than something the size of an X-wing. (Those two craft are not directly comparable due to vast differences in shape; I mention them merely for size.) At a certain point, the lifters required to get a craft out of a gravity well become impractical, if not outright impossible. With that in mind, you can see why large craft remain in orbit and use shuttles, and why they rarely (if ever) land on a planet.
Civilian craft can get away with a little more engine for their size than something that is full of weapons and fighter-craft while military vessels will sacrifice a bit of equipment that doesn’t fit their expected operational parameters (comfortable living quarters, indoor malls, swimming pools…) for things that help it do it’s job better (weapons, shield generators, maneuvering drives…). Something like a freighter, or cruise ship will reverse those priorities since they aren’t designed for combat. Their job is almost the opposite; land, get loaded with cargo/passengers, and lift off again while leaving the whole combat thing to ships that are made for combat.
So it’s not unreasonable to assume that even if the Star Destroyer cranked the power to the lifters up to levels only attainable with all emergency overrides locking out the usual safety measures, it just wouldn’t have enough to pull out of it’s dive, merely enough to hit the ground at lower speed. Why would it have enough lifters if it were never expected to land? There’s part of the equation.
Considering that Beskar (basically steel made from Mandalorian iron) can stop a light sabre, it’s not entirely out of the realm of feasibility that they have materials that can survive the heat of re-entry at a speed lower than what gravity would give the craft if unopposed by any sort of lifters with little/no ablation. And considering that Star Destroyers are military vessels, they likely have the best materials available. (“Best” often has cost considerations in addition to anything related to engineering though; even the most advanced mil-tech gear is made by the lowest bidder.) At the very least something that can withstand mega-power lasers should something take out the shield generators. Durasteel is a common choice as it’s reasonable strong while not relying on rare (thus expensive) ingredients like Beskar does.
With all that in mind, it seems to me that a large military spacecraft could fairly easily get sucked into a planet’s gravity well and hit the ground without burning up on reentry, hitting the ground hard enough to lose structural integrity in any number of interesting ways, or leaving a crater while kicking up enough dust to block out the sun for long enough to potentially kill any plant-life that the lower rungs of the food chain rely on, with all of the ripple effects that entails.
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“I am just saying unless in a galaxy far, far away they got much, much better at that, how could they be any better than NASA of today?”
NASA hasn’t made a pod-racer or hyperdrive yet, so I’d wager that the SW universe has slightly more advanced technology than 21st-century Earth. I’ve already named a couple of materials, one of which is common enough to be used for cookware.