^^^ in theory if both the design group and manufacturing group are employed by the same company then this would be the case but this is NOT how the bicycle industry works since most modern brands are design and R&D companies who then find a Taiwanese factory to make what they have designed.
You just need to look at who has driven the different specs to see that it is a design feature and not driven from the factory floor; Cannondale started with BB30. They were working in alloy so casting, machining and welding costs would be unaffected by the change (aside from tooling costs) but it allowed them to continue on the oversized tubing path. BB86/92 is driven by SHIMANO, a component manufacturer. BBRight is Cervelo. They don't manufacture anything, are heavily driven by design and have had other design driven frame features in the past proven problematic in terms of compatibility.
Pressfit30 is FSA, non? Another component maker. Specialized has their own BB spec (sort of) and is another design company. Trek is probably the only company who actually makes some of their frames that has introduced a spec that deviates from BSA .
I've worked in the bicycle industry since the mid 80's and I can assure you that what I have been saying has been told to me by several different frame manufacturers here in Taiwan. I also said that I wasn't given enough specifics to present concrete facts as to why this is the case but that I would after a series of factory visits I have scheduled in a few weeks.
Unless you are very, very familiar with carbon manufacturing methods it is impossible to speculate which process is more cost effective and why. Since you asked the question in the first place this clearly is not the case.
As for the shop floor, bike mechanics aren't even part of the manufacturing food chain. Having run several shops over the years I assure you that nothing is easier to service than BSA. I can swap out a BSA BB in less than 5 minutes and that includes removing and reinstalling the crank and the better part of a cup of coffee. Dealing with press in anything is a PITA, time consuming and has a much, much greater propensity for error.
You can choose to believe whatever you want to. Personally all the different specs give me a headache and I hope the industry will settle into two or three in a few years but in the meantime we'll continue to sell the various adapters needed to solve all the compatibility issues that are currently frustrating end users like yourself.