I suppose I do too.
While a lack of designed-in security is often the (sometimes disastrous) result, I posit that it's more about a lack of knowledge/education around good software security design/implementation than spec changes or scope creep.
If good security design practices were stressed when teaching software development (and/or included in resources for autodidacts) and given the same value as other development concepts, developers would incorporate those concepts from the start, rather than having to bolt them on (or not) later on.
As such, it's more about lack of knowledge/focus/value placed on security in software design/development.
Given the value of data that being secured with software, good security practices should be, if not just as integral, to software development as performance or UX design, it should at least be a consideration before prototyping.