One reason we might assume "engineers" tend to operate with better ethical frameworks is that (in Australia at least) you generally have to register for accreditation via a large organisation like Engineers Australia, IChemE, etc, to actually practice professionally. These organisations have standard codes of ethics that, if breached, can result in your removal and an inability to continue professional practice.

Naturally, software engineering has none of this, and in most cases explicitly doesn't want it.

But that's one reason I can think of.