You can’t thickness plane plywood reliably. The laminations are often too thin and will tear apart in low spots, or uneven glueing on the laminations will cause it to delaminate. It’s likely to make more of a mess than a usable product.

Then of course there’s the issue that not everyone owns a thickness planer, or they own small planers which can only feed in materials smaller than, say, 400mm wide which adds likely too many constraints for these projects.

Exactly this. The idea was to consume as much complexity in the design and keep the build process as simple as possible so as many makers as possible with only a few tools could work with the designs.