Because reasons.

See Herb Sutter's talk at CppCon 2015. At a given moment he asks the audience if they use any kind of static analysis tools.

About 1% of the audience said they were using one.

Which is why everything that requires external tooling, instead of being part of the language is easily ignored.

Would it help if the static analyzer were folded into clang itself and enabled with a flag (-Wanalyze?) rather than using a separate tool? That way it could just be a slight tweaking of build flags, and left enabled if it performs well on a given code base, rather than requiring extra work and thought.

Well it helped when XCode came out with integrated clang static analysis, vs with the largely ignored lint.

The main reason people avoid static analysis is excessive false positives.