Because LTO is a global decision while amalgamation (or unity builds) is a local decision. As a global decision, LTO can't be too slow and is subject to heavier

Because LTO is a global decision while amalgamation (or unity builds) is a local decision. As a global decision, LTO can't be too slow and is subject to heavier trade-offs. Combining files in this way is a local decision that you can just do if you know it would be a-okay and doesn't otherwise impact the whole build.