Why are flat feet "a bad thing for most people"?
If you look at the structure of the foot - particularly the mid-foot, you will see that the joints line up along an arch. The tendons that run under the foot - the posterior tibial tendon and the plantar fascia especially - are designed to support that arch and absorb shock. In the normal flexible flat foot those tendons are really stretchy - like a rubber band - so they can absorb shock pretty efficiently. In the European foot those tendons are not at all used to that kind of shock, so when they are stretched they tear - and when they tear they typically heal shorter, unless some therapy is used to keep them elastic. Children have flexible flat feet. But by the time a child reaches about age 12 those tendons should become more rigid to support the arch.
Note that I mentioned the "normal" flexible flat foot above. In Europeans a flexible flat foot is abnormal, whereas in some Africans the flexible flat foot is considered "normal" and functional. Sometimes it is called a "functional" flat foot.
By the way - "flat foot" is a term synonymous with "foot pronation" Flat feet almost always over-pronate.
me thinks i am thick today - i dont think i understood!