In engineering an idea that has been found to work in one place gets adapted in another. Also you often build reliability through adding layers of redundancy.
Evolution has no straightforward way for good ideas to be identified and reapplied to another system. It has to evolve again. Also if you have two fully redundant systems, there is nothing to stop deleterious mutations from disrupting one of them. The end result is that one of the redundant systems will become unreliable, and then the other one gets maintained by selection pressure. (This is also why creatures that live in a cave eventually become blind, and parasites lose their ability to use now unnecessary body parts. There is no engineering reason why this would happen, but is a natural result of selection pressures being eliminated.)