You want to test your code in the same environment that has generated the bug report. Since so much of Boost is header-only and inlined into your code, the two are irredeemably intertwined.
It's not that using the system-installed or user-installed Boost wouldn't work, it would, that's just a different workflow and a different scenario. It's a difference in philosophy and the two schools of thought don't seem to understand each other.
Plus, it's just plain easier to tell people "`git clone --recursive my-wonderful-repo` and run CMake."
For what it's worth, no company I have ever worked for has relied on system installed packages. Everything is either distributed in-tree or fetched from a package manager, by version, and usually as source. This is true even for the compilers. The intent is to keep the local build environment as similar as possible to the continuous build environment. Of course, that is not the only way to work, and it's not super convenient if one is working on smaller projects at home, so it's absolutely necessary to support the spectrum. -- chris