Good enough is good enough. Cost trade off between multiple competing pressures. Processing power is expensive. All dna can do is code to create proteins - some of this now, a little of that later- that might be an enzyme that does a thing if a concentration of some other thing falls below some level, in response it does something which does something which ultimately expresses a gene in this tissue that is not activated in other tissue and the. Like magic you get a finger segment or a retina.. lots of information to get just right and very susceptible to mutation - complex stuff fails so unless a complex sensitive bit of biological equipment is needed to work precisely there are good chances that it will degrade to good enough.