Going way out on a limb here, but my guess is that if you look at manufacturer A who makes 3, 4.5, 6, and 10kA breakers and manufacturer B who only makes a 10kA breaker that manufacturer A's 10kA breaker is going to be more expensive than manufacturer B's 10kA breaker. A's 3 or 4.5 will probably be less than B's 10, but the cost of making multiple versions will push all of A's costs up.
I've seen, and continue to deal with, the results of people optimizing designs in the past to use just the bare minimum; in fact we paid contractors bonuses 20+ years ago to spend as little as possible on as tight a design as possible. The added O&M costs since then probably exceeded the first cost savings within a few years and we keep paying and paying. Useful projects get put off simply because a $10k project will require many times that in infrastructure upgrades simply to make the necessary space available.
If you're designing a manufactured product that's going to be produced thousands or millions of times then by all means make it as cost efficient as possible, optimize the heck out of it. Particularly if you aren't going to be responsible for ongoing maintenance of it. But if you're building something for the ages, the guys/gals that come after you to maintain it will curse your name for years to come if you optimize it to the nth degree and leave no wiggle room.
If you can use a 6kA today because you calculate 5.8kA and then the utility upgrades their system and then the value calculates out to 6.1kA, how much have you saved in the long run by installing 6kA breakers instead of 10kA? Not going to happen you say, the utility won't need to make that change; well something will change. Maybe the laws will change and indoor grow operations for personal medical or recreational use will become legal and the neighborhood power consumption goes up by 50% and the transformers all become overloaded and get replaced with larger transformers. Things happen and overly optimized designs become obsolete much quicker.
Having too many times come after the guy that picked just the "right" size/rating/layout over the years, I get a bit testy; "oh, not again". The original question, and some of the follow ups, remind me of the design "logic" that ended up with a single section 600A MCC (lowest vertical bus rating is 600A) with two size 1 starters and a 60A feed to the MCC. The other four cells in the MCC were forever useless because that 60A feed couldn't carry anything else. The owner then paid much more to get a couple of motors added than they would have if the feed to the MCC had been 200A originally. But 60A was a lower first cost. Bah.
