It's because, once created, the item doesn't change. It never burps in the middle of casting, it doesn't have a headache, it won't get distracted if the maid knocks on the door in the middle of casting.
Etc. Etc.
You're describing something that a killer GM would do to players. There's little reason to say that someone casting a spell daily in his own home is under stress.
It's kind of a stretch to say on the one hand, magi are so incapable of working magic daily that they have to worry about burping inadvertently, the untimely knock of a maid, or the occasional headache (Harry Dresden might have something to say about headaches and working magic) and then on the other hand that they can perfectly (remember items don't need a roll to be completed, just time and vis) make an item that can do whatever is necessary.
So, under your understanding the warping from spells comes as a result of the daily difficulty of being interrupted during a 6 second window of time? I mean, that's 6 seconds out of 86,400. And if he has an item, that took 3-6 months to build, there's no risk, and it's effects don't create warping.
You don't have to be under stress to make tiny errors. Remember, it's not warping the caster, it's warping the recipient.
You spend 3-6 months perfecting something, and it's a lot more reliable than something you do in 6 seconds, because you have more time to iron out any kinks.
So, I'm reviewing Covenants, specifically trying to find Arthur's assertion stated earlier:
Arthur, can you cite this? The only thing I found about warping on Covenants, page 121 was this:
Spells have a bit of flexibility as they can subtract from Warping or Safety, whereas, from where I see it, items do create warping. Spells have a drawback in that they cannot provide a bonus when they are of the same Technique and Form combination as the lab work being worked upon, which is really a rather large problem, since magi will generally prefer to work in their TeFo specialty. That restriction doesn't seem to apply to items.
So, for an average magus without Flawless Magic they can get the same benefit if they spend 9 months mastering a spell and get an ability score of 2, and with Flawless Magic it's 3 months getting to an ability score of 2? I was looking for something much less subjective, and had some gravitas, or basis in RAW. I mean, in my home lab, I'm not going to have more than 1 botch dice, unless I'm suddenly under attack, and in that case, it would be entirely moot.
I should have said that spells might, instead of do, cause Warping.
The quote from p.121 specifically mentions "powerful effects", so I understand this to mean those of 6th magnitude or higher. For such powerful effects, both spells and items would indeed cause Warping if they affect the lab itself, rather than just the item. It also says that the effect may increase the Warping. This is left to the Storyguide's discretion.
You obviously know this, as it is taken into account in you subsequent comment. But just for reference, here is the second paragraph from p.122, under Spells for Laboratories:
Here's a idea for a magical defense system for a lab, magic stink bomb!
basically you cover a intruder with a FOUL stench!
Great if you want to uncover a enemy without necarrally killing him/her or to track said enemy to his/her employer!
It seems to me that in terms of lab bonuses vis costs the ring duration and scale is the biggest issue. I mean if I were setting up a laboratory with these rules one thing I would want is a stone walkway around the lab. Then I can walk the walkway, cast a spell with ring target and endurance, and it will affect the entire lab as long as the walkway lasts, without having to spend vis. If I were worried about security I would put it on the inside of the sanctum wall.
The Laboratory of Bonisagus is a lvl 45 effect so the ABSOLUTE maximum level this effect can have with sun duration is 55. I'd argue that it would be substantially easier to just create tools/equipment compared to a whole lab though and most likely less complex or comparable to creating a chainmail. Adding +2 or +3 for complexity seems a lot more reasonable and in line with other spells making it level 40 or 45. And it could create both tools and equipment of course, but with the obvious problems already pointed out with disappearing containers.
Interesting thought. Ignoring all the issues with finesse rolls and ritual effects. If an Effect like this where possible couldn't it actually have the consequence of reducing not increasing a lab's warping score.
Sure the tools are the target of a powerful magical effect in being created. But that's what 1 point? But since new ones are created every day it will never increase or at least never accrue more warping like every other thing in the lab.
There is a other issue with magical created tools in that they would have to penetrate any kind of magical resistance that the materials you use in your Lab work might have.
Non magical tools don't have this problem.