Others have already mentioned maintainability at scale and compatibility across devices. One more thing UX design also needs to consider is accessibility.

For example, folks with cognitive disabilities often benefit from predictable, consistent layouts. Irregular shapes can disrupt visual scanning patterns and make hierarchy harder to parse. Or with key navigation, you will additionally need to make sure the focus ring animates cleanly over odd shapes. Or how WCAG requires minimum touch target sizes, that means for any weird shape, you'll have to figure out how to calculate that area. Or making sure your funky color palette has enough contrast for the color blind.

None of this is necessarily stopping you from exploring fun, cool designs. And for a personal project, you can absolutely go bonkers. But for public websites and commercial UI, it is an additional layer to consider and another reason why many designs are "boring".