Is a hexagonal house feasible?
Building parts, building tools, building techniques, and building skills are all based on square walls. With building parts, you have things like bricks with 4 sides, wood sheathing with straight edges, not to mention studs, drywall, and most other building materials with factory edges. With tools, framing squares, levels, speed squares, and corner tools for drywall mud and paint, are all based on 90 degree angles.

However, the big reason to me are building techniques and skills. With 90 degree cuts, a top/bottom plate for framing the wall doesn't matter if it's upside down. You don't have to worry about which side of the framing the measurements are based on because it should be the same measurement. When checking that walls are aligned, builders use 3-4-5 triangles and measure the diagonals of a rectangle to ensure that they are square.

Finally, many homes are built on lots that often have parallel sides and homes are often built to maximize the utilization of these lots.

When building things that aren't square, there's also an increased risk for waste due to mistakes. Construct the framing with the 60 degree angle reversed and you'll likely have to throw out some of the framing. And for the exterior siding/finish, you'll end up with lots of cuts for bricks or custom corner pieces for siding. With the added materials and labor, this significantly increases the expense of building a home.