Why doesn't dendritogenesis continue in adulthood?

It is theorized that certain genetic disorders are caused by selection pressure and overdominance, e.g. a recessive mutation may boost IQ in heterozygotes despite being harmful or even lethal in homozygotes. Examples include Tay-Sachs or Gaucher's disease. See Ectopic dendrite initiation: CNS pathogenesis as a model of CNS development (Siegel et al., 2002).

The putative mechanism for elevated intelligence is reinitiation of primary dendrites on mature cortical neurons. My question is, if adulthood dentritogenesis promotes learning and boosts IQ why hasn't the human organism evolved to have common non-pathological alleles that do this already without the need for dangerous mutations?