This is not always correct. Depending on the roof construction and the climate you're in the attic may need to breathe (for example if part of the house vents into it), in which case blowing in loose insulation onto the attic floor is the correct solution.
But with climate change and how long roof lasts, lots of people need vents added or no longer need them.
I believe there's even some with a temp and/or humidity switch these days so it only opens when necessary.
Really tho, what matters is how much attic space we're talking here.
Mine was renovated before I bought my house, and while I can walk down the middle I can't really take a step either side. But I'm abnormally tall. And the last owners used it for a teenagers bedroom. If someone was like 5'8 the space would be a lot more usable.
But doing the bare minimum to make it "livable" drastically inflates the price even if you don't use it.
Long run it's worth it, so it makes sense to plan ahead and when things like this come up to do it as if it was already a living space.
I'm not saying rip the existing insulation out and do the roof/ceiling
I'm saying leave that, and add the insulation to the roof/ceiling.
If it has plywood floors, it's usable for storage, and these days theyre often eventually remodeled to become a bedroom drastically increasing value of the home.
So rather than remove the poor performing insulation in the attic floor, leave it and add more to help the attic become a living space eventually.
It's just planning for the future, and achieves the same result.
Even if it's sealed (it shouldn't be), the giant air pocket of slightly warmer air would be working as insulation as well.
So from outside to lower level, there'd be more r factor and it would be more energy efficient
Like, they're not "heating" it unless there's vents up there anyways...
You made a short comment, but there's a lot wrong with it.