I've spoken to various friends about this issue, and they've had the
same experience. I don't get why this seems to be so prevalent in the
home renovation / construction industry.

Because the vast majority of contractors out there are sleeze balls.

Contractors are no worse than customers. How'd you like to look at job after
job, spend hours on travel time and estimating, and later find out that the
customer did the job himself, using much of the info you or other
contractors gave him?

Hardly the same as getting ripped off by bad contractors. Just because
you give an estimate and suggestions to a customer doesn't mean he's
required to hire you.

However, you getting hired and paid by a customer DOES mean that you're
supposed to finish the job correctly.