Why are genes so faulty and diseased ?

We all have thousands of alleles, spread over 46 chromosomes. When we make a baby, baby gets 23 of those.

Copying errors aka mutations do happen. Everybody has about a dozen. Most of the time, if you have a faulty allele from one parent and a healthy analogous one from the other, you won't notice.

In nature, having a faulty allele from both parents reduces your chances of procreation.

In human society, they increase your chances of not getting sent to battle.  In some countries, people - healthy or not - are being encouraged financially to have a lot of babies...