Because it's yet one more special-cased thing to deal with – until now, Default was a regular trait, and there is no good reason to make it special. The motivation of making "default" functions out of thin air is weak at best, and misguided at most.
If anything, C++ ctors are an example of how not to do constructors. I don't think Rust should follow their example.
Also, there being 130 lang items is not at all a good reason for adding a 131st one. It's at most a sign that the lang team is taking lang items way too liberally. Instead of working towards removing special-casing (e.g. Box, which is a lang-item for historical reasons rather than necessity, i.e. it's essentially pure technical debt), it seems we are moving towards making even more types and traits into lang-items, which could otherwise be (and were) perfectly regular library types and traits.