A natural wetland is messy. It's full of fallen logs, debris, mud, fluctuating water levels, and sulfuric odor. Many plants native to King County wetlands are seen as weeds, like cottonwoods, alders, willows. A natural wetland has trees, at least on its borders, that provide shade. Developers or homeowners fear that big trees near their houses will grow up into hazard trees.
Designers tend to exclude trees and chaos in favor of flowering shrubs and attractive arrangements.
Some of these mistakes result from ignorance. Ten years ago, when the county first started mitigating, they didn't realize how shade-dependent many species are. No one thought about adding compost because native plants aren't fertilized - now we realize they use root mycorrhizae that transform minerals into nutrients. Mostly, no one realized that a mitigation is a construct, not an imitation. But most mistakes result from greed.