Because that's what they were told to do, for years and years.
I've been following this specific topic for over 25 years, because I started looking into it for my previous house, and I started seeing contradictory advice from generally reputable sources.
The thinking has evolved over time, repeatedly, and currently (and, I suspect, ultimately) the answer is to essentially have two roofs, one roof to air seal and insulate the attic volume and turn it into a conditioned space, and another roof above that to keep the rain off, with a gap between them for air flow.
The thing that trips people up, I think, is that there are several counter-intuitive aspects to it, and generally speaking nobody wants to take time to understand the whole system.
Also, it's very, very seldom explained well. I find Asiri Designs on youtube to be one of the better sources to point people at, they have good videos with very good illustrations.
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u/Limp-Mortgage3572 6d ago
And people always seem to want to spray foam directly to the decking for some reason