Well, first of all, it is important to understand that when we program item generation, we follow two principles:
1. The objective of the item is to make the Pokemon individually better, almost all the time disregarding how the rest of the team is composed (some exceptions to this rule could be Shedinja getting sash only if your team has defogers, sash on certain lead mons, abilities based on weather). The individual capabilities are measured on BST to determine most of the common items, like Choice items, Leftovers, Life Orb, etc. For a thorough revision of this rules, you can read the Gen 8 Random Battles competitive guide.
2. We try to give general rules to the code that generates sets. Exceptions must follow a clear and undeniable path of making a Pokemon better, otherwise we could clutter the code too much, making it too convoluted and difficulting manteniance and bug fixing. So the code states that if a Pokemon has Shell Smash, it will get White Herb.
I dont know from what tier did you take those stats, but remember that teambuilding metas have also 3 important advantages over Random Battles when optimising a Pokemon: they can make sure the Pokemon has a good team sinergy, they can build the Pokemon while having a rough but clear idea about what checks and counters they will face, and they can give the Pokemon perfect EVs on certain stats (Random Battles use 85 EVs on each stat, with some Exceptions like, in your examples, Polt gets 0 EVs on Atk to reduce Foul Play damage). Taking this on account, lets review your questioned Pokemon:
- Cloyster may be the closer one to work better than White Herb in Randbats. Still with the prevalence of Dynamax, which cannot flinch, it could come as underwhelming at the end of the day.
- Carracosta having HDB is clearly because it also can get enough EVs to ensure to outspeed most of its checks and counters of the tier to afford risking its defenses for keeping Sturdy. We cant assure that and Carracosta never was an extremely fast sweeper in Randbats, although it usually needs more than 1 hit to get koed, even with hazard damage.
- I assume Polteageist runs Sash because its frail, but in a similar way as Carracosta, it wants that second live chance to pull off ShellSmash and then Strength Sap whatever they have in front. The issue here for Randbats would be that it heavily relies on having hazard control, which you cant assure in Randbats (much less taking on account we cant EV hazard removers to be bulky and make sure to eliminate the hazards before the sweeper enters).
I hope I answered specially why we dont neccesarilly follow item usage on teambuilding tiers (although its not like we ignore them whatsoever either).
Even then these are only my takes on it, and Randbats sets are decided by a broader team, the Random Battles Room Auth under the direction of code-owner Annika. Feel free to continue this discussion, and/or provide calcs for set suggestions or critics on the Sets thread.