Because we don't yet understand software engineering. There has been insufficient empirical study of what yields maintainable/performant/what-have-you code, of what sorts of abstractions are "good" for maintenance/reuse/etc., at what's needed to reliably predict a program's resource requirements, and many software developers balk at design tools that restrict their style to detect certain errors as early as possible, like type systems.
Software development is not yet an engineering discipline for all of these reasons, and more.