Mostly because Eugenics doesn't understand genetics or breeding and has little appreciation for the broad horizons and deep histories of our DNA. Replacing these little topics and their boring scientific rigor with hubris certainly won't end well.
Unless you are from some remote village in Finland or Tennessee, it's unlikely you have a suitable percentage of inbreeding to have a genetic concentration that would reliably produce equal or better offspring. Instead, each breeding will be mutts diluting any potential benefit of lineage further.
But crossbreeding can also have advantages that can revitalise stale line breedings as seen in the hybrid vigor found in crossing two lines such as the Labradoodle.
Hybrid vigor is magical and can produce offspring better than both parents. Sometimes. So too can the mutt strategy. Sometimes.
However, if you don't understand the concept of hybrid vigor being a terminal breeding, you will be quite frustrated in attempting to extend your line when you discover there is no line. Even if you put out the dough to send your kids to vaccine free Waldorf schools, you will not erase the likely reality that you are a lucky mutt, not even a hybrid, simply a product of random breeding. As a result, you are a terminal breeding yourself.
If you really want intelligent children, you would be miles ahead to buy them from a reputable breeder with a track record of proven test breedings; making a family isn't a DIY weekend project - it takes centuries.
