To raise a child takes time, and even though lust and good harvests might speed up the *!*
, breeding is a rather slow process.
Population is a resource, much like gold or metals. During ancient times chinese warlords force-moved farmers from one province to another. This was probably done by other ancient empires as well. After all, population was the most important resource of an empire. We realized that a game mechanic that allowed moving populations would cause all inhabitants to be gathered into capitals. Not a very good solution.
I like the fact that it is impossible to increase the population. It is the only non renewable resource in dominions.
Last remark: There are three kinds of spells I can think of that would increase population: teleport thousands into your province, create thousands out of thin air or force grow thousands. I do not like the first two options and the Last one strikes me as inefficient if you want to create a farming population. I prefer to keep crossbreeding for war only purposes.