The longer answer is that there are no declarative build tools because all the build tools aim to support all the insane build requirements of existing C or C++ projects. At that point embedding a turing complete scripting language actually makes for simpler build descriptions than some massive declarative system trying to handle all the corner cases.
Of course, that raises the question: why don't any C or C++ build tools limit themselves to some sane subset of build requirements? That, I don't have a good answer for.
1. No, using Turing-complete scripts does not prevent information extraction and meaningful automation. You can ask the program to dump useful information, e.g. targets, compiler flags, etc. That's what rizsotto/Bear does for Makefile. CMAKE_EXPORT_COMPILE_COMMANDS is another example.
2. I don't really think you can do anything further with declarative build languages, unless it is a really limited one like JSON or XML. Meson (a relatively advanced one in the space) advocates for non-Turing-completeness, but you still cannot, for example, modify meson build files reliably using an external tool other than a text editor.
3. Complex build configuration usually requires non-trivial computation and/or dynamic dependency graph building. Turing-completeness gives you a possibility and you don't need to wait for build tool upgrades.
Comparing Meson to CMake, I find the niceties of Meson are usually not inherently non-TC. That is, you can theoretically reimplement a nicer CMake with all the niceties of Meson, while still being TC.
All a rigidly non-turin-complete build tool means is the day will come where you have to throw it away and rewrite everything in a more powerful language, probably at the last minute and with a bunch of fires taking light around you.