On the one hand, I hear ya - at least as far as quantities are concerned. That'd make my life (as an estimator) a LOT easier. And there ARE standards for many such things - how many nails in a sheet or sidewall sheathing, for example. Trouble is twofold, at least. For starters, many of the "standards" (building code minimum requirements are a GREAT example) are set so low that NO reputable builder adheres strictly to them. Those requirements are just too lax. Nor do I design to them, for the same reason. I'll "over-design", thank you very much. My customers seem to agree...
To top that off, there are as many ways to tackle a specific "building issue" as there are builders doing it. How do you "guesstimate" waste factors, for instance? A straight-forward simple roof is going to generate a lot less waste than a particularly cut-up one. That's why I said to enlist the help of a local estimator. A good one (I consider myself to be one such, and apparently my boss agrees, since I still have a job) will know the particular builders and their practices, and can estimate accordingly.
The "science" just ain't that exact, there's a pretty big bit of "art" involved. Which is why the CA programmers (or anyone else) could never be expected to be able to cover all the bases. If they did/could, the program would become so complex that no one could use it!
And that's just the "what to use" part of the equation. "How much does it cost" is even more unpredictable. Many building products - lumber & sheathing especially - are commodity items. Prices change DAILY, even hourly! The ONLY way I can lock your price, let's say for a project to begin in May, is to buy a futures contract on today's market. That'll be 2 rail-car loads of studs, and you don't begin to need that many. Same for lineal 2x materials. Same for sheet goods. How many houses are you gonna build???
Sure, the company I work for publishes item pricing. It'll be good until I have to buy the next batch. Then it WILL change. Same way gas prices change from day to day. Not saying you have to like it, but it is what it is, and it's not going to change regardless of whether you like it or not. Even "non-commodity" items - siding, windows, doors, trim, etc... change pricing periodically.
A "typical" house has probably something on the order of 15,000+ different items. Oh, and they can be vastly different items, from house to house. Do YOU want to maintain that database of pricing?