I’m never surprised by the incompetence or laziness people, even “pros”, do to unseen parts of construction. Many times they know it will work long enough to be someone else’s problem, which is quite unfortunate.

I see the same with tile. Installers skipping waterproofing is quite common. DIY folks trying to make it look pretty so they can sell the home for top dollar, etc. It often creates a much bigger problem for the next person to deal with.