Attics of houses today are not designed for storage anyways.
The insulation piles quite high above the structural joists and you would end up sitting your boxes on top of the insulation, compacting it and reducing it's thermal barrier properties.
All roof trusses are engineered and have a significant amount of diagonal and horizontal structure running around that would also make it difficult.
I've been in a lot of attics. I sometimes have a hard time just getting myself up there and navigating my way around.
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