I think 3D printing walls could potentially make isolation cheaper by building it directly into the structure, similar to how wasps build their hives.

I am less convinced about funny shapes as in the end you still want your furniture etc. to fit, and I know from painful experience that renovating an old house with walls that aren't in a 90° angle can be a real PITA.

It’s not all about making blobby organic shapes that are hard to furnish though. You can make square interiors for rooms and in addition have voids strategically placed in the concrete to create chimneys and reduce solar gain, for instance, or your example of incorporating insulation.

This is the kind of discussion I’d like to have though. Sure, one of the reasons dome homes don’t catch on is the awkwardness of using circular spaces. But that’s no reason to think the exterior, wall-interiors and ceilings have to be orthogonal.

You can have a squared off room with an epic organic vaulted ceiling designed to create natural ventilation, for instance. Such things are currently expensive because they are custom construction, but what becomes economical when you don’t have to make concrete forms?

Some things are square (like the interior walls) because it’s just more usable, as you mention. Many other aspects of stick-built structures are square just to facilitate cost-effective construction, usually labor and material saving, not because it’s the best way when considering for the whole lifespan of the building.

You answered your question:

modern houses are shaped the way they are because of ease of construction and cost effectiveness…

The freedom to construct free-form structures does little to alter the efficiency. Round rooms have more square footage per wall area than square and spherical shapes have more volume than cylinders or cubes per square foot of exterior. What they gain in theoretical thermal efficiency they lose in other ways. A good example is a bed. A square bed fits better in a square room, and most beds are square.

The majority of the freedom is on artistic expression or layout customization. No matter how perfect the layout might be for an owner, the more custom a house is the less adaptable it is for life changes, new/future owners, and changes of use. One of the biggest advantages of a boring-ass box is its universality and customization options.

Edit - sorry for not answering your question directly. There are ways we could leverage 3D concrete printing - such as ribbed/strengthened below-grade foundations (basements).

Wasp out of Italy is doing the beehive or wasp nest look – Wasp 3D prints eco-homes from local raw earth (Kirsten Dirksen) – should be easier to adapt for hexagonal packing (at least give you some flat walls for beds and bookcases)

This isn't new...

All poured houses have been a thing for decades.

And while rounded edges would be hard to frame up perfectly and might need manually smoothed, the same is true for this as well.

But since this doesn't have perpendicular support, I'm assuming it's either incredibly slow, or the bottom walls will be significantly thicker than the top of the walls.

another reason for the boring boxes is the reality that you want to build something that others will value and want to buy from you later. sure you could make a sculpture of your house to look like your favorite dog, but who would buy it from you? maybe if its a masterpiece of universal beauty sure, but just for the sake of being different? go check out the values of cars that have customized kit body work done versus stock. if you dont care about resale value and are willing to make custom furnishings then hey go nuts, aim for architectural digest.