I'm not sure what companies you've worked with, but the answers to your questions are very easy and come with experience. There's a universe of testable things. You'll never have 100% coverage (this is a software development law). Even if you were to do everything right, and build incredible automated deployment and testing systems, at best you'll end up reducing but never entirely eliminating fragility. To even get to that level of perfection would require a tremendous amount of overhead and time that most enterprises don't want to invest in the systems they build.

It's akin to trying to constantly remodel/add additions to a building. You may decide that you want new floors, but when you tear up the carpet you realize there's tons of water damage that was being covered.