It is important to recognize that we are now talking about 399 BC. 200 years have transpired since Lehi left Jerusalem. The amount of population growth possible in that length of time is remarkable. Apparently, the only thing that was checking an uninhibited population explosion was the unending conflict with the Lamanites. The phrase multiplied exceedingly, and spread upon the face of the land gives us just a hint at how large the population could have been at this time.
Also, because of their relative righteousness, the people have become rich. Their society is complex, their buildings and machinery of fine workmanship. Some sense of these accomplishments can be realized with the archeological ruins left in Central and South America. The society had become complex.