feudalism produces groups of people who have bound others to property to do that farming while they are on campaign, that's what knights were. a modern levee en masse is much more disruptive to agricultural production, because it doesn't tap classes who are already expected to go on a campaign. one shouldn't even assume because a certain peasant area was required to produce so many levy soldiers that this would be an excessive draw on the labor pool either--it bad times it might've, but in others a handful of third sons weren't exactly missed, and those men entering armed life was expected and prepared for in advance.