Mostly because it's hard to do sound related detection with the games systems.
[...]
They're locked into certain systems due to it being based on D&D rulings. So that's why we have no stealth checks for performing actions while stealthed
I'm curious to know what D&D you've played, Taril ^.^ Certainly not any system that looks closely like 5e.
- Other games have incorporated sound as an element of stealth before, even contemporary ones, and it's effective enough to do without the need to overcomplicate it. Solasta incorporates sound in their stealth system very effectively. In 5e, your Dexterity (stealth) check contains both the traditional elements of Hide and Move Silently in it; these were more explicit in older editions,but the elements still subtly exist in the background in 5e and are both contained in the single stealth roll (certain magic items call out their affects applying to stealth if the stealth specifically involves hiding, or if it involves moving silently, for example).
- BG3 does not follow much of 5e's rules for stealth at all - it completely perverts them and is built 90% on the existing D:Os2 game engine's stealth mechanic. There is no need for a stealth check at any point and you have zero risk of detection at all, as long as you don't step in the sight cones. This is
Divinity's system, with nothing of D&D in it at all... so claiming that they were somehow constrained by the D&D system, and that's why stealth is as it is is extremely disingenuous of you. You
know that's not true, so why say it?
A system being broken beyond recognition and stripping all meaningfulness from the system doesn't stop being the case just because "it's not necessary"; the system is broken, and someone who would like to meaningfully engage in stealth CANNOT, because of how broken it is. They either don't use it, or they break the game and push the cheating "I win" button that it supplies. There's no way to actually engage with legitimate stealth play in any proper way.
In terms of breaking stealth with actions: from a game design perspective, if you haven't made a completely inaccessible spaghetti engine, it's a straight forward design concept to put markers and identifiers on abilities or groups of abilities, as to whether they break stealth automatically, or grant fresh opposed checks to all in perceiving range. This isn't a "too hard basket" issue, and shouldn't have been considered one for any dev team, sensibly speaking. Any action that interacts with another object, and any spellcasting, should provoke a fresh stealth check if you do it while hidden. Certain spells will have flags for being 'loud' or 'visible' - which will not prompt a check but automatically end your stealth. In other cases, whole classes of action (attacking, for example), will be in the automatic break category, and not require special flagging individually. The player should be able to clearly see what things will break stealth, and which won't, or will force a check. This is basic level design theory, and given that they had the freedom to make this game the way they wanted (they were definitely not constrained to make it a 5e D&D game, since the most it can claim is a loose "inspired by" - or more pointedly "Painted to look like, because we want the visibility, brand and license to pimp our other games to a bigger audience" - that one is word of Swen himself), it's virtually criminal that we were given what we were, instead.