Hello,
No particular reason, extrude works fine on the walls and will leave you with a solid. I guess it goes to show there's more than one way to come come up with the desired result in MoI. If you need to create walls that do not follow a linear path Sweep is one way to get that result while using Blend (shown in green) to connect those walls with desired continuity, as shown here in this image. As far as the solid issue goes I think I must have not selected the correct edge to create the upper plane, I try to remember to hide the curves when doing those sort of surfacing operations so I am able to pick the correct surface edges, that's my guess anyway.
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