Americans are incredibly parochial (which would be o.k. if they were not also convinced to be the center and paradigm of the whole world). While it's obviously true that some selected football, basketball and soccer players would be very good in some track events (mostly sprints and jumps), there are also events like distance running, discus or high jump were this is less likely.
Furthermore, track and field is practiced all over the world. The preferred and most lucrative professional sports are rather different in different countries (for most of the world soccer dominates but the US and several other countries are exceptions to this). Still, this makes the talent pool for t&f huge because no matter of baseball, soccer or hockey is your national team sport, track will be practiced as well. Whereas there is very little baseball in Europe or hockey in Africa etc.